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MAP READING 

AND 

TOPOGRAPHICAL SKETCHING 



MAP READING 

AND 

TOPOGRAPHICAL SKETCHING 



BY 
EDWIN Rf STUART 

PROFESSOR OF DRAWING, UNITED STATES MILITARY ACADEMY, 
AUTHOR OF "TOPOGRAPHICAL DRAWING," ETC. 



First Edition 



McGRAW-HILL BOOK COMPANY, Inc. 
239 WEST 39th STREET. NEW YORK 



LONDON: HILL PUBLISHING CO., Ltd. 
6 & 8 BOUVERIE ST.. E. C. 

1918 



A^ 



>*< 



«0 



Copyright 1918, by the 
McGRAW-HILL BOOK COMPANY, Inc. 



OCT 30 1918 



/ 



CI.A506409 



PREFACE 

In this book the author has attempted to record for the 
benefit of others something of the results of thirteen years of 
experience in the practice and teaching of topographical surveying 
and sketching. 

There is no short cut to excellence in topographical work. 

Good maps result from the patient and careful labor of men 

thoroughly instructed in the means and methods of topographical 

representation. 

West Point, New York, 
October, 1918 



CONTENTS 

Page 
PREFACE v 

AUTHORIZED ABBREVIATIONS xi 

CHAPTER I.— Introductory 1 

CHAPTER II.— Map Reading. 

^^Scales of Maps — Reading Scales 4 

Scales of Maps — Reading Scales — Methods of Stating Scales on 
Maps — Comparison of Methods of Stating Scales — Construction of 
Graphical Reading Scales — Examples — Rules for the Construction 
of Reading Scales — Problems. 

CHAPTER III.— Map Reading (Continued). 

j/Scaling Distances on Maps 14 

Methods of Scaling Distances — Straight Lines — Meandering Lines — 
Accuracy of Distances Scaled from Maps — Measurement of Dis- 
tances along Roads — Problems — Other Exercises — -Measurement of 
Artillery and Infantry Ranges — Map Positions of Points on the 
Ground — Triangulation Points — Points along Roads — Interior 
Points. 

CHAPTER IV.— Map Reading (Continued). 

(Working Scales 22 

Measurement of Distances in Surveying and Sketching — Determina- 
tion of Length of Stride — Determination of the Rate of a Horse — 
Construction of Working Scales — Problems — Problems Involving 
Erroneous Scales — Example — Problems. 

CHAPTER V.— Map Reading (Continued). 

I/CONVENTIONAL SlGNS 29 

Conventional Signs on Topographical Maps — Special Military 
Symbols — Color Conventions — Relation of Conventional Signs to 
Map Scales — Familiarity with Conventional Signs — Exercises. 



viii CONTENTS 

Page 
CHAPTER VI.— Map Reading (Continued). 

" Representation in Relief 45 

Methods of Representing Ground Forms — Hachures — Contours — 
Contours on the Ground — Contour Lines on the Map — Datum Level 
— Vertical Interval between Contours — Methods of Stating Slopes 
— Relation between Slope and Distance between Contours — 
Relation between V.I. and Map Distance between Contours — 
Construction of Slope Scales — Examples — Problems in Construction 
of Slope Scales. 

CHAPTER VII— Map Reading (Continued). 

^Contours (Continued) 57 

Forms of Contours — Valley Contours — Spur and Ridge Contours — 
Summit Contours — Col or Saddle Contours — Representation of 
Slopes by Contours — Uniform Slopes — Concave Slopes — Convex 
Slopes — Exercises — Models and Sand Box — Contour Problems — 
Construction of Profiles — Problems — Visibility — Visibility Problems. 

CHAPTER VIII.— Map Reading (Continued). 

Map Reference 71 

Designation of Points on Maps — Map Sheets — Map Reference — 
Reference Lines — Designation of Map Sheets — Sheet System of 
Reference — Location of Points in Grid Squares. 

CHAPTER IX.— Map Reading (Continued). 

Directions and Positions on Maps. 

u Orientation 77 

Direction — Determination of Direction — The Magnetic Needle — 
Approximate North by Polaris — Approximate North by the Sun — 
Orientation — Orienting a Map by Compass — Orienting a Map by 
Identified Points — Location of Points on the Map — General 
Accuracy of Maps — Representation in Plan and in Elevation — 
Identification of Points on the Ground— Finding Your Own Loca- 
tion on the Map — Relation of Map Scale to Detail — Horizontal 
Detail — Vertical Detail — Use of Compass with Map — Compass 
Bearings — Marching by Compass. 



CONTENTS ix 

Page 
CHAPTER X.— Topographical Mapping. 

- Instruments and Methods 91 

Mapping Methods — Standard Sketching Equipment — Sketching 
Board and Tripod — Alidade — Clinometer — Accessories — Improvised 
Sketching Equipment — Using the Standard Sketching Equipment — 
Sketching Paper — Assembling the Sketching Board — Carrying the 
Sketching Board — Setting up the Sketching Board — Orienting the 
Sketching Board — Alidade — First Use — Second Use — Clinometer — 
Slope Board. 

CHAPTER XI. — Topographical Mapping (Continued). 

.^Traversing — Selection of Control Points — Con- 
tour Problems 105 

Traversing with the Sketching Board — Location by Resection — Con- 
tour Problems. 

CHAPTER XII.— Topographical Mapping (Continued). 

•'Sketching from a Located Point 115 

Selection of Ground for Practice — Scale and Vertical Interval — Con- 
trol Traverse — Analyzing Ground Forms— Specific Example — 
Measurements from the Point — Checking Results — The Level of the 
Eye — Sketching by Estimation — Flat Hill-tops — Valleys — Hillsides. 

CHAPTER XIII.— Topographical Mapping (Continued). 

^Estimation of Distances, Mounted Sketching, 

Etc. Lettering 122 

Estimation of Distances — Speed in Sketching — Mounted Sketching 
— Trench Sketches — Lettering on Sketches — Alphabets — Hints for 
Practice. 

CHAPTER XIV.^Fanoramic Sketching 131 

Purpose — Perspective — Crest Lines — Distance Effect — Conventions 
— Sketching Pad — Practice in Panoramic Sketching — Making the 
Sketch. 



AUTHORIZED ABBREVIATIONS 



A. 


Arroyo. 


L.S.S. 


Life Saving Station 


abut. 


Abutment, 


L.H. 


Light House. 


Ar. 


Arch. 


Long. 


Longitude. 


b. 


Brick. 


Mt. 


Mountain. 


B.S. 


Blacksmith Shop. 


Mts. 


Mountains. 


bot. 


Bottom. 


N. 


North. 


Br. 


Branch. 


n.f. 


Not Fordable. 


br. 


Bridge. 


P- 


Pier. 


C. 


Cape. 


pk. 


Plank. 


cem. 


Cemetery. 


P.O. 


Post Office. 


con. 


Concrete. 


Pt, 


Point. 


cov. 


Covered. 


q.p. 


Queen-post, 


Cr. 


Creek. 


R. 


River. 


cul. 


Culvert. 


R.H. 


Roundhouse. 


D.S. 


Drug Store. 


R.R. 


Railroad. 


E. 


East. 


S. 


South. 


Est. 


Estuary. 


s. 


Steel. 


f. 


Fordable. 


S.H. 


School House. 


Ft, 


Fort. 


S.M. 


Saw Mill. 


G.S. 


General Store. 


Sta. 


Station. 


gir. 


Girder. 


St. 


Stone. 


G.M. 


Grist Mill. 


str. 


Stream. 


i. 


Iron. 


T.G. 


Toll Gate. 


I. 


Island. 


Tres. 


Trestle. 


Jc. 


Junction. 


tr. 


Truss. 


k.p. 


King Post. 


W.T. 


Water Tank. 


L. 


Lake. 


WW. 


Water Works. ;. 


Lat. 


Latitude. 


W. 


West. 


Ldg. 


Landing. 


w. 


Wood. 



MAP READING 

AND 

TOPOGRAPHICAL SKETCHING 



CHAPTER I 

INTRODUCTORY 

Map reading and topographical sketching are essential tools 
of the military profession. Topography has an important influence 
over every military operation. Marches and tactical movements 
of troops, are planned on maps, and the orders for military opera- 
tions must in general be based upon some specific map. Such 
orders can be understood and intelligently carried out only by 
reference to an identical map. Thus throughout the entire chain 
of command in the military service a thorough understanding 
of maps is required. 

Since topography is a governing consideration in military 
operations, it follows that topographical information which is 
unimportant in the civil uses of maps may be of the utmost 
importance for military purposes. 

A military map is a map showing the topographical informa- 
tion which is important from a military point of view. Much 
of this detail is temporary in character, such as information 
concerning annual crops, details as to the character and con- 
dition of roads and bridges, etc., and therefore not shown on 



2 MAP READING AND TOPOGRAPHICAL SKETCHING 

ordinary topographical maps. Further, the minute detail, such 
as the smaller irregularities of the ground, fences, cuts and fills, 
etc., important from a military standpoint, can be shown only 
on maps of comparatively large scale. 

Minute. and temporary topographical detail means high first 
cost of survey and frequent revision, hence satisfactory military 
maps of any except possibly a very few important areas will not 
be available at the outbreak of the war. Practically all nations 
have completed or are working on general topographical maps 
of their territory. These are mainly for civil uses, and of too 
small a scale to be satisfactory for military purposes. The 
general topographical map of the United States is being prepared 
by the U. S. Geological Survey, and is on a scale of about one inch 
to one mile. It is fairly representative of the general character 
of maps available at the outbreak of the war, or in operations 
where the forces are shifting position rapidly. 

But modern experience shows that war has a tendency to 
settle down into a series of protracted struggles more or less 
stationary in character. When such a situation arises, time 
becomes available for the expansion of the scale and detail of 
the maps of the areas involved. Furthermore, the character of 
the operations resulting from such a situation demands the use of 
maps in great detail and of a high degree of accuracy. Military 
forces may therefore be called upon to expand the scale of exist- 
ing maps or even to create new maps for their own use during 
active operations. 

Thus there is required in the military profession not only the 
ability to read maps, but also the ability to make them. The 
ability to read maps must be exercised by all grades down to 
include the non-commissioned officer. The bulk of the work of 
making maps will fall upon those specially trained and organized 
for this purpose. But every non-commissioned officer and 
junior officer should be able to make a sketch which will show 
accurately the detail of his own surroundings. 



INTRODUCTORY 3 

Study and practice will give the ability to interpret in a purely 
mechanical way the topographical symbols on a map. But 
only by learning to make maps can be acquired that intimate 
understanding necessary to enable one to interpret and evaluate 
the topographical information embodied in a map. One who 
has had experience in topographical work in the field will know 
from the map where the topographer has been and where he has 
not been; will know where the map is accurate and where only 
approximate; will know how much generalization and inaccuracy 
exist even in the best topographical maps; will know how much 
the generalization and inaccuracy are affected by the scale of the 
field work; and will be able to recognize by the topographical 
" expression " of a map the work of a competent topographer and 
equally unerringly the work of an incompetent one. 

The ability to read maps does not require an extensive course 
in topographical surveying, but it does require the study of 
topographical features on the ground itself, and practice in the 
representation of these features with the degree of accuracy and 
detail possible on various scales. Such practical work can be 
carried out satisfactorily with comparatively crude equipment, 
because the faculty to be cultivated is that of representing the 
topographical features about a given point. The more or less 
mechanical manipulation of instruments by which a point is 
located is of much less importance. 



CHAPTER II 

MAP READING 

Scales of Map^ — Reading Scales 

There are two essentially distinct elements in the art of read- 
ing a map: 

1. The ability to interpret in a mechanical way the information to 

be found on a map. 

2. The ability to weigh the topographical evidence given by a map. 

Mechanical interpretation of the information recorded on a 
map requires familiarity with the scales of maps; familiarity 
with all of the conventional topographic symbols used in record- 
ing data on maps; and familiarity with the conventional methods 
employed in representing the form of the surface. This knowledge 
of map reading can be acquired by indoor theoretical work, 
preferably supplemented by study of models in connection with 
maps as an aid to visualization and clear comprehension of the 
ground forms represented. 

To be able to weigh the topographical evidence given by a 
map, practical experience in the work of representing ground 
on a map is necessary. Only by work on the ground will it be 
thoroughly understood how utterly impossible it is to locate by 
measurement every feature shown on a map, and how much of 
the data even on the best of topographical maps rests upon the 
estimate of the topographer. Even in the largest scale maps the 
representation is still very small compared to the area represented. 
The difficulties, generalizations, and approximations of topo- 

4 



MAP READING 5 

graphical representation can be thoroughly appreciated only 
by one who has familiarized himself with the methods used in 
making maps, and has actually attempted in practice to represent 
ground on paper. In this practice, a sharp distinction will be 
encountered in the accuracy of representation in plan as compared 
to that of the representation of ground forms. 

Before attempting the representation of ground on paper, 
however, familiarity with the conventional methods of repre- 
sentation is necessary. This portion of the subject, which covers 
also the mechanical element of map reading, will accordingly be 
discussed first. 



Scales of Maps 

One of the most frequent uses of a map is to determine the 
distance between points on the ground represented. In order 
to furnish this information, distances between points on the map 
must bear a constant relation to the distances between the same 
points on the ground. This relation of map distance to ground 
distance is known as the scale of the map. The term has exactly 
the same significance as in all other drawings.* 

It is well to emphasize the idea that the scale of a map is a 
simple matter of ratio of distances. The apparent complexity 
of the subject arises from the fact that, even in the case of large 
scale maps, the map distance is a matter of inches, whereas the 
ground distance is a matter of yards or even miles. Consideration 
of the subject of scales of maps therefore requires a conception 
of the ratio between very short and very long distances. 

* The earth is spheroidal in form, and the absolutely accurate representa- 
tion of its surface on a plane is impossible. Many forms of map projection 
have been devised to reduce this distortion to a minimum and it is negligible in 
the projections in ordinary use. The discussion of the subject of map pro- 
jections and all other difficulties in map making resulting from the curvature 
of the earth belongs to treatises on geodetic surveying. 



6 MAP READING AND TOPOGRAPHICAL SKETCHING 

The first difficulty that presents itself is due to the fact that 
our conception of short distances is in inches, of moderate dis- 
tances in yards or as a fraction of a mile, and of longer distances 
in miles. Thus a certain short distance may be readily recognized 
as 3 inches, but only in map reading and map making are we 
forced to learn to recognize 3 inches as 21 \ 20 of a mile. Nor 
does 1,584,000 inches convey a definite impression of distance 
until the mind has become accustomed to thinking of a mile as 
63,360 inches, and we learn that the given distance is 25 miles. 

The second difficulty lies in the fact that in topographical 
surveying distances are measured, not in inches or miles, but in 
feet or rods; and in topographical sketching distances are mea- 
sured in still other units, in strides ( = double paces) of the sketcher, 
or by timing a horse at a gait of known rate. 

This confusion of units makes it necessary to use two dif- 
ferent classes of scales for maps: 

1 . Reading scales for the purpose of ascertaining the ground distances 
corresponding to distances measured on the map; and 

2. Working scales for the purpose of plotting on the map distances 
which have been measured on the ground. 

Reading Scales. 

A reading scale is a graphical scale which, when applied to a 
map, reveals to us the distances between points in units that we 
understand. For one accustomed to our units of measure, the 
reading scale may give short distances in feet or yards, but must 
give longer distances in miles and fractions of miles. Reasonable 
familiarity with the metric system enables one to use a kilometer 
scale, but such use is generally accompanied by a menial calcula- 
tion of the distance in miles by multiplying by f or .6. 

The ordinary form of reading scale is shown in Fig. 1, in which 
the major graduations read miles and the extension quarters and 
eighths, these being the familiar subdivisions of a mile. A read- 
ing scale of kilometers would show kilometers as the main divi- 



MAP READING 7 

sions, with the extension subdivided decimally to agree with the 
metric system. Reading scales of yards are useful mainly in 
connection with infantry and artillery fire, and therefore applicable 
generally only to large scale maps. The main divisions represent 
100 or 1,000 yards, and the extension is subdivided into decimal 
parts of the main divisions. 



a * 







A 



5 Mi. 
d 



Fig. 1. — Typical Reading Scale. 

To permit a more accurate measurement of distances, recourse 
m&y be had to the construction of a diagonal scale. Such a scale 
is used principally for the decimal subdivision of the smallest 
division of the regular scale. The principle of the construction 
of a diagonal scale is shown in Fig. 2. Parallel to the line of the 



100 50 



100 



200 



300 400 500 Yds. 



■ 



Fig. 2. — Diagonal Scale. 



main scale are ruled ten lines with any convenient equal spacing. 
The ten subdivisions of the main scale are repeated on the last 
line, and the extension completed by joining the subdivision 
points as shown. A scale so prepared has a least reading ^ of 
the smallest subdivision of the main scale. 

A diagonal scale is of little practical value in measuring dis- 
tances from maps, because its use assumes a higher degree of 
accuracy than most maps possess. For all ordinary military 
purposes the least reading of a direct scale is sufficient. 



8 MAP READING AND TOPOGRAPHICAL SKETCHING 

Methods of Stating Scales on Maps. 

All maps should either have printed upon them their own 
reading scales, or else show the data necessary for the prepara- 
tion of a reading scale. 

The data with respect to the scale of the map may be given 
in one or more* of three ways: 

1. As a ratio between the map distance and the corresponding 
ground distance, expressed as a fraction called the representative 
fraction (abbreviated R.F.), written j^ , ~^- Q , ^7^, 
etc. The numerator of the R.F. is the distance on the map, 
and the denominator is the corresponding distance on the ground, 
both expressed in the same units. The first fraction means 
that 1 unit distance (inch, centimeter, etc.) on the map represents 
10,000 of the same unit distances (inches, centimeters, etc.) 
on the ground. To those accustomed to the metric system, it 
means that 1 centimeter on the map represents 10,000 centimeters 
or 100 meters on the ground, and enables the regular centimeter 
scale to be used for scaling distances from the map. In our own 
units, a scale of 10 1 000 means that 1 inch on the map repre- 
sents 10,000 inches =833i feet, 277 £ yards, or .1578 mile,— all 
of which are odd distances bearing no relation to our usual con- 
ception of distances. 

2. As a simple statement of the relation between map distances and 
ground distances, both expressed in some familiar unit: as 1 
inch = 1 mile, 6 inches = 1 mile, 1 inch = 100 yards, etc.- If the 
statement is given in some unit with which we have not a work- 
ing familiarity, it must be translated into our units before it is 
of practical use. 

3. As a graphical scale drawn on the map and so subdivided that 
the usual units in which ground distances are measured are 
represented in map distance by the subdivisions of the scale. 
Thus on a scale of 1 inch = 1 mile, the subdivisions of the graphical 
scale would be 1 inch in length, but the subdivisions would be 
designated 1, 2, 3, etc., miles. 



MAP READING 9 

Comparison of Methods of Stating Scales. 

The persistence of three different methods of expressing the 
scale of a map is explained by the fact that each has certain 
advantages not possessed by the others. The R. F. is a statement 
of ratio which is independent of the unit used, and enables any 
one to construct a reading scale for the map in units with which 
he is familiar. 

The actual ratios used as the scales of maps are dependent 
upon the units familiar to those who make and use the maps. 
Thus the convenient ratios for those familiar with the metric 
system are all decimal, as 5i00 ^, io.ooo> 20 00c 4o.ooo> so.ooo 
10O ' 00l) , etc., because these ratios give directly the relation 
between the small units of the ordinary metric scale in which map 
distances are measured and the large units in which ground dis- 
tances are measured. In fact, a representative fraction such 
as those written above gives directly all the information necessary 
to use an ordinary centimeter scale for the purpose of reading 
distances in kilometers from the map. 

For the same reasons that cause the use of decimal ratios in 
the metric system, we are forced to use in our maps a system of 
representative fractions running j^, r^J^, 21 ,i ao , 31> S 80 > 
63 ,3 60 > etc., because these are the ratios which obtain when 
inches on the map represent familiar fractions of a mile on the 
ground, thus enabling us to use an ordinary scale of inches as a 
reading scale for our maps. 

From the above discussion, it is seen that the ratios expressed 
by the representative fractions of maps are inseparably connected 
with the units of measure familiar to those for whose use the 
maps are principally intended, and that those ratios convenient 
for one system are inconvenient for another. 

However, if the representative fraction is recorded on a map, 
it furnishes the data for the construction of a reading scale in any 
system of units of measure, and is therefore generally regarded as 
an essential piece of information to be entered on a map. 



10 MAP READING AND TOPOGRAPHICAL SKETCHING 

The simple statement of the relation of map distance to ground 
distance merely tells how to use an ordinary scale for measuring 
distances on a map. The simple statement is simple only as 
respects the system of units of measure to which the scale of the 
map is conveniently related. Thus a statement that the scale 
of a map is 1 inch = 1 mile is good news to one provided with an 
inch scale and familiar with ground distances in miles, but it does 
not help much to know that the scale is one centimeter = one 
kilometer unless we have a centimeter scale and are familiar 
with ground distances in kilometers. 

It is manifest that a graphical scale whose subdivisions repre- 
sent ground distance units with which we are familiar is, after all, 
the only reading scale that is convenient for our use. Such a 
scale is more convenient if detached from the map, but there are 
two good reasons for printing a graphical scale on a map. One 
is that it will not be lost or missing when wanted, and the other 
is that it forms a record of the scale of the map which remains 
correct if the map is enlarged or reduced by photographic means. 
If the scale is given either by R.F. or by simple statement, the 
data are falsified by photographic enlargement or reduction, hence 
it is very desirable that the map shall bear on its face a graphical 
scale, even though the scale is also stated or the R.F. given. 

To be convenient for our use, a map must bear a graphical 
scale in our own units, or must be drawn to a scale conveniently 
related to our units. In the latter case, an ordinary foot rule 
forms the desired detached scale for use in measuring distances 
on the map. But if neither of these conditions be fulfilled, then 
it is necessary to construct a graphical scale suitable for the 
map to be used. 

Construction of Graphical Reading Scales. 

If the scale of the map is in some unfamiliar unit, it is neces- 
sary that the data with respect to the scale be in some form that 
makes it possible to determine the R.F. of the map. Thus if we 



MAP READING 11 

assume a Russian map with a graphical scale of versts printed 
on it, we could measure directly the length of the divisions repre- 
senting versts, but could proceed no further unless we happened 
to know that a verst is 3,500 feet. However, the difficulty of 
obtaining a great deal of useful information from a map with the 
names printed in Russian characters suggests that the discussion 
be confined to the more usual case of maps based on the metric 
system. These maps practically invariably show the repre- 
sentative fraction of their scale. If not, remembering that one 
meter =39.37 inches, a careful direct measurement of the graphi- 
cal kilometer scale will serve to reveal the R.F., especially as the 
denominator is certain to be some multiple of 1,000, except in 
the scale R.F. 2 J 00 . 

Example 1. Let us assume that the R.F. of the map has been 
determined to be 80 * 000 , showing a scale smaller than 1 inch = 
1 mile. Let it be required to construct a reading scale of miles. 

Solution. In this case it will be desirable to construct a 
reading scale with 1 mile as the main division, and showing 10 
miles on the main scale and 1 mile on the extension, or 11 
miles total length. Eleven miles = 11 X 63,360 inches = 696,960 
inches. With R.F. 80 1 000 , the map length of 11 miles would be 
696,960 -f- 80,000 = 8.712 inches, which is the total length of the 
scale to be constructed. Draw a line and lay off by scale 8.712 
inches as nearly as practicable. Divide this by the usual oblique 
geometrical construction into eleven equal parts, and subdivide 
the left-hand division into eighths by the same construction. 
This construction is shown (not to scale) in Fig. 3. The equal 
divisions on the oblique line are laid off by an inch scale and 
projected back on the line to be subdivided by lines parallel to 
the line B C. 

Example 2. Assuming a map scale of 20 * 000 , let it be required 
to construct a reading scale of yards for this map. 

Solution. One yard on the map represents 20,000 yards on 
the ground, hence 1,000 yards will be represented by f-§- of an 



12 MAP READING AND TOPOGRAPHICAL SKETCHING 

inch = 1.8 inches, hence we select 1,000 yards as the main division 
of the scale. A reading scale showing 6,000 yards will therefore 
be 10.8 inches long. By a construction similar to that shown 
in Fig. 3, divide a line 10.8 inches long into six equal parts. Sub- 
divide the .extension into ten equal parts. The main scale will 
show five divisions each representing 1,000 yards, and the exten- 
sion 1,000 yards with a least reading of 100 yards. If a diagonal 
scale be constructed as in Fig. 2, the least reading will be 10 yards. 



8712 




Fig. 3. — Division of Scales. 



Rules for the Construction of Reading Scales. In general, to 
construct a reading scale: 

1. Determine the R.F. of the map. If this determination results 
in a near approximation to a decimal ratio or to one of the frac- 
tions representing the inch-to-mile ratios, it should be assumed 
that the actual ratio is the nearest decimal or inch-to-mile fraction. 

2. Select as the main division of the scale a convenient unit, as 1 
mile, 5 miles, 100 yards, 1,000 yards, such that the main divi- 
sion will be not less than about | inch in length. For large scale 
maps, the main scale and the extension may each represent 
1 mile. 

3. Determine a convenient number of main divisions of the scale 
such that, with one division added for the extension, the total 
length of the scale will be between and 1 2 inches. 



MAP READING 



13 



4. Calculate the total length of the scale. 

5. Subdivide into the proper number of divisions, and subdivide 
the extension into convenient fractions of the main divisions. 

6. Mark the divisions of the scale properly. 



Problems in the Construction of Reading Scales. 

Problem 1. Construct a reading scale of miles and fractions of miles for 



Problem 2. 
Problem 3. 

Problem 4. 
Problem 5. 



a map scale of 



20.000 

Construct a reading scale of yards for a map scale of 



j. 

5.0 00' 

Construct a reading scale of yards for a map scale of 6 inches 
= 1 mile. 

A kilometer reading scale is shown on a map. The distance 
representing 10 kilometers measures scant 4 inches. Con- 
struct a reading scale of miles and fractions. 

A certain distance is measured on the ground and found to 
be 3 miles. The corresponding map distance is 4f inches. 
Construct a reading scale of yards. 



CHAPTER III. 

MAP READING— Continued 

« 

Scaling Distances on Maps 

Methods of Scaling Distances. 

Straight Lines. Straight-line distances may be measured by- 
applying a graphical reading scale directly to the map, or by 
measuring the distance with a pair of dividers or on the edge 
of a piece of paper, and then applying the length so found to a 
graphical reading scale. 

Meandering Lines. Meandering lines may be measured in 
several different ways: 

(a) Directly by applying the scale successively to short lengths, 
each of which approximates to a straight line, and summing 
the total distance measured. 

(b) The total length may be accumulated on a pair of dividers by 
setting the dividers at the initial point and measuring the first 
portion approximating a straight line; then swinging the rear 
leg of the dividers into prolongation of the next portion of the 
line to be measured and setting the rear leg on the paper; then 
extending the forward leg to the new forward point, and so 
continuing until the total distance is accumulated. Then apply 
the dividers to the scale. 

(c) By a process similar to (6), successive distances may be accu- 
mulated along the edge of a sheet of paper, and the total dis- 
tance applied to the scale. 

(d) For continuously curving lines, a thread or string may be made 
to conform to the line, and the total length thus determined 
applied to the scale. 

14 



MAP READING 15 

(e) An instrument called a "map measure" is sold by instrument 
dealers. It is provided with a contact wheel which when 
rolled over the map, records by a pointer and dial the distance 
in inches passed over. The map measure may be used on 
straight lines, but is particularly convenient for measuring 
sinuous lines. 

If the scale of the map is in inches-to-the-mile, the map measure 
reveals the ground distance directly. For other scale ratios, 
the map measure must be rolled backwards over the divisions 
of the graphical scale to determine the ground distance. In 
this process, the map measure must be rolled over such num- 
bers of even divisions that from the dial it becomes evident 
that one more division will cause the pointer to pass the zero. 
Then it is rolled over the extension scale until the pointer reaches 
zero to determine the fractional part of the reading. 

Accuracy of Distances Scaled from Maps. 

Measurements of distances on maps are made for two principal 
purposes : 

(a) Along roads for determining the distances between towns, the 
points which marching columns will reach at given times, and 
for various purposes in connection with the movements of troops 
and transport. 

(b) Along straight lines for the purpose of determining ranges for 
artillery and infantry fire. 

Measurement of Distances along Roads. As a general rule, 
the location of roads may be considered the most accurate data 
shown on a map, for the reason that convenience in surveying 
work of any kind demands that the roads shall be located and 
plotted accurately to serve as a basis for the rest of the work. 
Even in topographical sketching, the roads are plotted as accu- 
rately as sketching methods permit, and road distances on such 
sketches may be relied upon as being within a maximum limit 
of error of about 2 per cent. 



16 MAP READING AND TOPOGRAPHICAL SKETCHING 

Although the distances along roads are thus generally reliable 
in all classes of maps, the military purposes for which distances 
are scaled from the roads do not demand a high degree of accuracy. 
An error of a quarter or half mile in a distance of ten miles is 
of little importance in connection with marches of troops or move- 
ments of transport, because there are so many other elements 
of uncertainty in such operations that great accuracy of calcu- 
lation is unnecessary and undesirable. 

Problem 1. From Plate A, what is the road distance from the center of 
the bridge over the Red River to a point opposite the north- 
east corner of the reservoir? 

Problem 2. From Plate A, what is the distance by road from the bridge 
over Indian Creek east of the Water Works to the school 
house (S. H.) at Weston? 

Other Exercises. Use maps of as many different scales as may be avail- 
able. Select on roads points which could be easily identified 
on the ground. 

(a) Measure the road distance between points. 

(b) Compare the distances between points by various routes. 

(c) Find how long it would take to travel from one point to 
another at an assumed rate. 

(d) Assume that a column of troops 1 \ miles long with leading 
element at a certain point begins a march at 2\ miles per 
hour along a selected road at 7.15 A. m. Find the time 
when the leading element of a column 2 miles long would 
have to leave another selected point in order to cross 
the line of march of the first column just before the first 
column reached the cross road, or just after it had passed. 

Measurement of Artillery and Infantry Ranges. The measure- 
ment of ranges from maps presents an entirely different problem 
from that of measurement of road distances. Two factors enter. 

(a) The accuracy of the measurement as a mechanical process; and 

(b) The accuracy of the location and identification of the points on 
the map between which the measurement is made. 



MAP READING 17 

The. mechanical part of the measurement of the distance 
between points marked on the map offers little difficulty, assum- 
ing that the map is in good condition and lying flat on some 
suitable surface, and that the points between which the distance 
is to be measured are both on the same map sheet. It is then 
only necessary to measure the straight-line distance carefully 
and apply this distance to a carefully constructed reading scale. 
Since it is desirable to ascertain range distances as accurately 
as possible, it is preferable to use a diagonal scale for the purpose. 

However, maps in use in the field require special care to insure 
their good condition. On all maps that are fit for use for the 
purpose of measuring ranges, it may be assumed that the data are 
as accurately plotted with reference to the lines of the map pro- 
jection as possible. If the separate squares or rectangles of the 
map determined by the map projection lines be made to conform 
to an accurate reproduction of the same projection lines on a 
suitable and as nearly as possible unchangeable surface, the map 
will be in the best condition possible for use. This is accomplished 
by preparing an accurate reproduction of the map projection lines 
on a sheet of zinc. The lines are scratched on by a sharp steel 
point. The map is now cut into the squares defined by the projec- 
tion lines and each square is carefully pasted within the proper 
area defined by the projection lines scratched on the zinc. 
This insures that the worst distortion in the whole map shall be 
that due to the application of a single small section of the map 
in its proper place on the sheet of zinc. 

Furthermore, the method just described permits the assembling 
of portions of different map sheets into a continuous map, the parts 
of which are as accurately related to each other in direction and 
position as if they had been on the same map sheet. Maps so 
mounted are frequently used, particularly for artillery purposes. 

Assuming a properly mounted map, the distance between two 
points on the map can be measured accurately. Whether or 
not the distance thus measured represents accurately the distance 



18 MAP READING AND TOPOGRAPHICAL SKETCHING 

between two points on the ground depends upon the accuracy 
with which the map positions of the points have been identified, 
because the map distance will be in error if the assumed map posi- 
tion of either point is wrong. 

The accuracy of ranges measured on a map thus depends upon 
the accuracy with which the map positions of points on the ground 
are identified, and this in turn depends upon a knowledge of how 
maps are made. 

Instead of measuring ranges directly from the map, they may 
be calculated from the coordinates of the points. 

Map Positions of Points on the Ground.* 

All points on a map are not located and plotted with the same 
degree of accuracy. It is therefore desirable to know which 
points are located exactly and which only approximately. 

Triangulation Points. In general, all formal surveys are con- 
trolled by a series of points located very accurately by triangu- 
lation. These triangulation stations are located at points from 
which a wide view can be had, and are consequently to be found 
on the tops of prominent hills. The stations themselves are 
marked permanently on the ground so that they can always be 
identified. On the resulting maps, the triangulation stations are 
marked by points with small triangles drawn about them. Any 
point so marked on a map may be relied upon absolutely as being 
accurately plotted in position. Any point near a triangulation 
station should be referred to the triangulation station by proper 
measurements if the accurate map location of the point is desired. 

Points along Roads. Connecting the triangulation points of 
a survey are run a series of traverses covering usually the roads 
offering the best and most direct routes between stations. These 
traverses are corrected by the known positions of the triangu- 

* This paragraph should be read again after some practice is had in map 
making. 



MAP READING 19 

lation points, and constitute the next most reliable data on the 
maps. Traverses are then run covering the remaining roads, 
corrected to fit the traverses between triangulation stations. 
All of these traverses are usually run by instrumental methods, 
and are to be relied upon in order of nearness to the triangulation 
stations. 

. Although the roads themselves are thus reliable in their loca- 
tions, it is necessary to be cautious in identifying points along 
the road. If a house or bridge is plotted, it is safe to assume 
that a measurement has been taken to it and that it has been 
accurately plotted. But a topographical feature such as a spur 
of a hill or a patch of woods cannot be relied upon, because it is 
probable that it has been located only approximately. 

The only certain method, therefore, in locating and plotting 
the position of a point on a road is to measure on the ground the 
distance from some other point which can be positively identified 
on the map and ground, and then set off this distance on the map. 
On battle maps for the use of artillery, points along all main roads 
traverses should be described and listed with sufficient data to 
enable the points to be located both on the ground and on the map. 
Such a list will enable other points on the map to be located by 
suitable measurements from these known points. Descriptive 
lists of points on the road traverses are actually issued for use in 
connection with most, if not all, of the European battle maps. 

Interior Points. For points in the interior areas, it is neces- 
sary to exercise still more caution in accepting the plotted posi- 
tion. In making maps, it is impossible to locate all points by 
actual measurement. Thus only a few points on a stream course 
are located and the stream is then drawn in by estimating the 
meanderings between located points. Many other topographical 
features must of necessity be located by estimation from known 
points and lines. When familiarity has been gained by practical 
experience with the methods used in making maps, it is possible 
to judge with considerable accuracy just where the topographer 



20 MAP READING AND TOPOGRAPHICAL SKETCHING 

went and what points he located by actual measurement, but 
there will still be the question of whether specific points which it 
is reasonably certain that he located accurately are actually 
plotted on the map. Suppose the point to be considered is the 
plotted position of a house at a distance from the road. If the 
house is visible from two points on the road, which were probably 
used as stations on the road traverse, it is quite certain that the 
house has been located by intersection, and therefore its plotted 
position corresponds in accuracy with a point on the road traverse 
itself. Furthermore, it is certain that the house itself can be found 
on the ground. 

But if the point be a hilltop, then the accuracy of its location 
will depend to a large extent on its visibility from points on the 
road traverse, and further upon whether there is on or near its 
summit some object such as a tree, bush, or rock which might 
have been used as a point of sight for intersections. If so, that 
object was probably located accurately by intersections, or a rod- 
man may have been used in locating the point, but there remains 
the difficulty that it was not plotted on the map and its map 
location can only be judged by its relation to the top of the hill 
itself. In such case there may be a substantial error in judging 
the exact map position of any point on the top of the hill. 

A knowledge of the methods of map making is thus essential 
to a correct judgment concerning the map location of any selected 
point on the ground. Such knowledge will lead to caution as to 
accepting points seen on the ground. The safe course generally 
is to plot the map position of any point by making the necessary 
measurements on the ground to establish its relation to some 
point or points which may be safely assumed to have been accu- 
rately located by the topographer, and which are actually plotted 
on the map. Then the desired point can be plotted accurately 
on the map. 

So far as battle maps are concerned, the above discussion 
applies principally to points within our own lines, and refers 



MAP READING 21 

especially to the plotting of positions of batteries and machine 
guns. 

Ranges are measured from these points to the targets, which 
lie behind the enemy's lines. As a matter of fact, practically 
all the details within the enemy's position are plotted from air- 
plane photographs, and a target becomes known if it is discovered 
on. these photographs. In that case its position can be plotted 
accurately on the map from the photograph. There are also 
secret means of determining the positions of targets within the 
enemy's lines, but these cannot be described. 

It must be assumed that the map positions of targets are 
plotted as accurately as possible from the best information avail- 
able. Any errors in their map positions must be compensated 
for by corrections resulting from aerial observation of fire. 



CHAPTER IV 
MAP READING— Continued 

Working Scales 

In the process of making maps, it is necessary to be able to 
plot conveniently and accurately distances which have been 
measured on the ground. This involves the construction of a 
graphical scale showing map distances corresponding to the units 
of measure used in making the map, called a working scale. 
A working scale is in fact a graphical reading scale, differing in 
units but not in principle from the reading scales already de- 
scribed. 

Measurement of Distances in Surveying and Sketching. 

In topographical surveying, distances are measured on the 
ground in feet by the stadia or steel tape, sometimes (though 
rarely) by the surveyor's chain in rods and fractions. In mili- 
tary sketching, distances are measured by the dismounted sketcher 
in strides (double paces), and by the mounted sketcher in minutes 
and fractions by timing the horse at a gait of known rate. The 
unit of distance used in survey work is fixed. The length of 
the sketcher's stride -or the rate of his horse must be determined 
before starting work, and should be checked from time to time 
while work is in progress. 

Determination of Length of Stride. The length of the stride 
is determined by pacing over a measured course, counting each 
time the right or left foot strikes the ground. From the measured 
length of the course and the average number of strides deter- 
mined by pacing the course a number of times, the average length 

22 



• MAP READING 23 

of the stride is computed. The length of the stride varies with 
the height of the man, but the general average is about 64 inches. 
Although the process is thus simple, a considerable degree 
of care is necessary to secure satisfactory results. What is 
desired is a general average length of stride under the varying 
conditions to be encountered in sketching in the field. The 
necessary precautions are as follows : 

(a) Select a course which includes slopes not exceeding about five 
degrees, and lies partly on roads and partly across country. 

(6) Measure the course with a tape. Measurement along the sur- 
face will be sufficient. 

(c) Pace the course several times in each direction. 

(d) Guard against errors in counting the strides. A pace tally is 
made and sold by dealers, but its use is not recommended. The 
usual type is not entirely reliable, it forms an additional piece 
of equipment to be carried around, and it has the serious dis- 
advantage of not being made of non-magnetic metal and is for 
this reason objectionable as likely to influence the magnetic 
needle of the sketching case. Learn to count always on the 
same foot. It is well to keep track of the hundreds of strides 
upon the fingers. 

(e) Avoid being influenced by others who may be pacing the same 
course. Do not pace alongside or close to another person 
on account of the tendency to keep step. 

(/) Do not try consciously to keep a uniform length of pace, but 
take a natural gait. As a sketcher you will be thinking of 
your work insofar as counting strides will permit, and will 
give no thought to the length of pace. 

(g) Pace the course at the same gait that you will use in sketching. 
The length of the stride is affected by the speed at which you 
travel, and sketching is hard work involving much walking. 
Try to take the gait you would use in a long walk. 

(h) Do not try to force the length of stride either up or down hill. 
What you are trying to secure is a general average stride length 
applicable to distances on slopes as well as on the level. 



24 MAP READING AND TOPOGRAPHICAL SKETCHING 

From the data determined under the above conditions, com- 
pute the average length of the stride in inches. 

If desired, a single course may be laid out on a slope of about 
10 degrees, and its horizontal length determined. Pace over 
this course up and down hill, keeping a separate record in each 
direction. These data should not be used in determining the 
average length of the stride, but in determining a percentage 
correction to the number of strides on steep slopes. Thus, if 
the average stride length is 64 inches, and the average horizontal 
stride length is GO inches going up a 10-degree slope, it will 
be necessary to subtract iV of the number of strides taken in 
going up this slope to obtain the correct number of strides to 
plot with a 64-inch stride scale. 

The reason for not using steep slopes in determining the- 
average length of stride is that, as a general rule, pacing on 
steep slopes is avoided whenever practicable in sketching, and 
it is better to apply a correction in the few cases when pacing 
on such slopes is necessary. 

Determination of the Rate of a Horse. When used in sketching, 
the horse will invariably travel on roads. For this reason, a 
portion of road conforming to the average of the country in 
grades and condition should be selected and measured as a rating- 
course. The rate at a walk, trot, and gallop is desired in terms 
of minutes per mile of travel. The important gait for sketching 
is the trot. 

It is to be remembered that the rider and not the horse is 
responsible for the rate of travel. If left to himself, the horse 
will proceed at variable speed, and will travel faster coming- 
home than going in the opposite direction. Each horse has, 
however, a natural gait at a walk and trot which can be main- 
tained by the rider more easily than any other. It is these 
natural gaits which arc (;_> be sought and maintained in rating 
the horse. 

Having identified these natural gaits, the rider times the 



MAP READING 25 

horse over a measured course 1 to 2 miles in length, pref- 
erably with a stop-watch. The course should be repeated three 
or four times each at a walk and trot, to determine the average 
time per mile at each gait. The gallop is almost never used in 
sketching and hence may be omitted in rating the horse. Suitable 
alternation of walk and trot should be used in order to prevent 
undue tiring of the horse, which would affect the gait. 

From the data secured as above described, the time per mile 
in minutes and fractions is computed for both the trot and 
walk. 

Construction at Working Scales. 

For purposes of illustration, it will be assumed that the length 
of stride has been determined to be 64.5 inches, and that the 
time of a horse has been determined to be 13| minutes per mile 
at a walk and 1\ minutes per mile at a trot. 

The best method of procedure is to find by calculation the 
exact map distance corresponding to some suitable whole num- 
ber of measuring units. The latter number is to be so chosen 
that the actual map length of the working scale will be about 
6 inches. Lay off the map distance on the line representing 
the scale, and on an oblique line suitable even divisions to rep- 
resent the measuring units. Complete the scale by the method 
shown in Fig. 3. 

Example 1. Required to construct a working scale of 64.5- 
inch strides for use in sketching on a map scale of 6 inches = 1 
mile. 

Solution. The stride length being 64.5 inches, 1,000 strides 
will be 64,500 inches, or f-ftfl^ of 1 mile. The map length of 
1 ,000 strides will therefore be lt;H° X 6 = 6. 1 1 - inches. On the 
line representing the main scale lay off 6.11 inches, and divide this 
distance into ten equal parts by the method illustrated in Fig. 3. 
Subdivide the left-hand division into ten equal parts, and add one 
division on the right to complete 1,000 strides on the main scale. 



26 MAP READING AND TOPOGRAPHICAL SKETCHING 

The main divisions of the scale will be 100 strides and the sub- 
divisions of the extension 10 strides. Number the divisions. 

Example 2. Additions and corrections are to be made on 
a map, scale 20 ^ 00 . Construct a time scale for a horse which 
trots a mile in 1\ minutes for use in this work. 

Solution. The scale of the map in inches to the mile is 
2o!ooo ~3-168 inches. Hence a scale showing 2 miles or fifteen 
minutes of time of the horse will be of convenient length. The 
map length of 2 miles is 3.168X2 = 6.336 inches. Lay off this 
distance and subdivide it into 15 equal parts, each of which 
will represent one minute at the trot. Add one division for 
the extension, and divide it into quarters. The subdivisions will 
represent fifteen seconds. 

Example 3. Construct a 64.5-inch stride scale for use in 
plotting positions on a map of scale 10 ,ooo- 

Solution. The length of 1,000 strides is 64,500 inches, and 
the map length of 64,500 inches on a scale of 10 * 00 is f o.ooo 
= 6.45 inches. Lay off 6.45 inches and subdivide into ten equal 
parts, each of which will represent 100 strides. Add one division 
and subdivide it into ten equal parts, each representing 10 strides 
for the extension. 

Example 4. Construct a working scale for stadia readings in 
feet for use in plotting points on a map of scale 5 ,q 00 - 

Solution. On this scale, 1 foot on the map will represent 
5,000 feet on the ground; hence the scale length of 3,000 feet will 
be §t^-§-§ of 12 inches = 7.2 inches. Lay of 7.2 inches and divide 
into three equal parts. Subdivide the left division into ten equal 
parts and then construct a diagonal scale according to the method 
illustrated by Fig. 2. The least reading from the diagonal scale 
will be 10 feet. 

Problems in the Construction of Working Scales. 

Problem 1. Construct a working scale for 62.5-inch strides for use in 
sketching on a scale of 12 inches = 1 mile. 



MAP READING 27 

Problem 2. Construct a working scale of 64-inch strides for work on a 
map of scale 1 inch = 100 yards. 

Problem 3. Construct a walk scale of minutes for a rate of fourteen 
minutes to the mile on a map of scale i — . 

^ 30.000 

Problems Involving Erroneous Scales. 

A great variety of problems may be stated involving the 
effect of the employment of erroneous working scales upon the 
reading scales of maps. However the data may be given, they are 
always susceptible of being reduced to the form that the stride 
length or the rate of the horse was supposed to have a certain 
value at the time the map was made, whereas it was in some 
way discovered later that the value was erroneous. 

No matter how these problems are stated, it is always 
possible to calculate the erroneous stride length or the rate of 
the horse used in making the map, and the later ascertained true 
value of these units. The original erroneous construction of 
the working scale can be reproduced, thus showing the actual 
distances plotted on the map corresponding to strides or min- 
utes actually counted in making the map. The revised data 
can always be used to calculate the true values of the strides or 
minutes, and the correct number per mile. The plotted length 
of this number of strides or minutes can be determined by inter- 
polation on the construction of the false scale, and its projection 
on the main scale will show the true map length of 1 mile, from 
which a correct reading scale can be constructed. It is safer to 
solve these problems in this way than to attempt to establish 
by reasoning the ratio between the false and true scales, because 
of the inverse ratio between length of stride and number of 
strides per mile. 

Example. The stride length was calculated from a meas- 
ured course and found to be 63 inches. A working scale of strides 
was then constructed and used in making a map on a scale of 
6 inches = 1 mile. Failing to check accurately on some known 



28 MAP READING AND TOPOGRAPHICAL SKETCHING 

distance, the course was repaced and the correct stride length 
was found to be 65 inches. What was the correct scale of the 
map? 

Solution. If the original construction of the stride scale 
be reproduced, it will show that 63 6 3 3 60 = 1,006 strides were con- 
sidered as 1 mile and actually became 6 inches on the working 
scale. The later data show that 63 g 3 5 60 =975— strides actually 
covered 1 mile. The plotted length of 975 strides is the true 
value of 1 mile on the map, which is 1 9 7 5 6 
5. 8+ inches. 



( = fi) of 6 inches = 



Problem 1. Over a measured course believed to be 6,235 feet long, the 
length of stride was determined to be 64 inches. The result- 
ing stride scale was used in making a map on a scale to'.Wo'- 
It was later discovered that the correct length of the course 
was 6,325 feet. What was the true scale of the map? 

Problem 2. The rate of a horse had been determined to be 1 mile in 



3 inches = 1 mile. Two weeks later the same working scale 
was used in sketching, but the horse was slightly lame. 
By mile posts on a road traversed, it was noted that a mile 
was covered in eight minutes twenty seconds. What was 
the correct R. F. of the sketch made that day? 



CHAPTER V 

MAP READING— Continued 

Conventional Signs 

Conventional Signs on Topographical Maps. 

A topographical map suitable for military use should show 
the railroads, roads, trails, paths, large streams, and canals, all 
of which are important as means of communication; cities, towns, 
villages, and isolated houses, important as indicating possible 
sources of supplies of all kinds and shelter for troops; smaller 
streams, lakes, ponds, large springs and wells, important as 
sources of water supply for troops and animals; woods, orchards, 
growing crops, and grass land, important as indicating sources 
of supplies of fuel and possibly of food, and still more important 
as offering concealment for troops or possible obstacles to the 
movement of enemy troops; and the form of the ground, which 
is all-important as affecting the disposition of troops for battle. 

All of these topographical features are represented on maps 
by appropriate conventional signs. These conventional signs 
are designed to form a clear and easily interpreted record of the 
topographical data. Simplicity of signs is desirable as a matter 
of economy of time in drawing, as well as on account of clear- 
ness of the resulting map. 

On account of the relatively small scale of even the so-called 
" large-scale " maps, it is desirable that the conventional signs 
should be capable of readable execution in small space, in order 
that a maximum of data may be entered upon the map. 

29 



30 MAP READING AND TOPOGRAPHICAL SKETCHING 

A standard system of conventional signs is desirable in order 
that any map that comes to hand may be used without danger 
of confusion or misinterpretation. The need for uniformity in 
representation is such that practically all nations have adopted 
standard systems of conventional signs which are required to 
be used in the preparation of all maps issued by the different 
departments of the government. 

Although each nation has thus a standard system of con- 
ventional signs, the desire for uniformity has not yet secured 
the adoption of any international standard, and the conventional 
signs differ on maps of different countries. There is a general 
agreement in the signs for the principal features, such as roads, 
streams, cities, etc., but even these differ in detail, so that it 
is necessary to know the particular conventional signs used 
on the map under consideration. Very frequently the standard 
set of conventional signs used is printed either on the margin 
or on the back of the map itself, where reference is easy. If 
not, it is always possible to procure in sheet or pamphlet form 
a complete set of the conventional signs used in any set of topo- 
graphic maps. 

For our own maps, a standard set of conventional signs has 
been adopted by the United States Geographic Board for use by 
all map-making departments of the government. These signs 
have been published in War Department Document No. 418, 
which can be obtained from the United States Geological Survey 
Office, Washington, D. C. For convenience, the principal ones 
of the topographical conventional signs are repeated in Figs. 4-15 
inclusive, at the end of this chapter. 

Special Military Symbols. 

In order to record upon maps the position of troops, bat- 
teries, machine gun emplacements, searchlights, and the multi- 
tudinous other things that play their parts in modern military 
operations, it has been necessary to expand very greatly the 



MAP READING 31 

previous list of special military symbols, and further additions 
are necessary at frequent intervals. So rapidly is this list ex- 
panding that it is necessary to procure an up-to-date set of the 
special signs for the particular maps being used in order to guard 
against mistakes. For that reason, none of the special military 
symbols is repeated here. 

Color Conventions. 

There is a great gain in the clearness of maps by the use of 
different colors to represent different classes of information, as 
will be seen by reference to Plate A. Three colors are generally 
used on small scale topographical maps on which no vegetation 
is shown, and four on large scale maps showing vegetation. On 
battle maps, two additional colors are used — one to show the 
military positions and works of the enemy and the lettering per- 
taining thereto and another for those of our own troops. 

The color conventions are as follows: 

Black for the works and constructions of man, such as railroads, 

roads, telegraph and power lines, bridges, ferries, fords, cities, 

towns, villages, buildings, fences (except hedge), state and other 

municipal boundaries, for the lines of map projection and map 

reference, and also for all lettering except such as pertains to special 

military symbols. 

Brown for natural and artificial ground forms, including contours 

and contour numbers, cuts, embankments, levees, sand banks and 

dunes, gravel banks, rocks, and for the embankment sign in the 

rice symbol. 

Blue for all water features, such as oceans, bays, lakes, ponds, 

streams, canals, aqueducts, pipe lines, drainage and irrigation 

ditches, marshes, swamps, and glaciers. 

Green for all vegetation symbols. 

Special colors, one for all enemy forces and military works and 

lettering pertaining thereto, and another for our own. 

These color conventions have been regularly used when the 
conditions attending the publication of maps permitted, as for 



32 MAP READING AND TOPOGRAPHICAL SKETCHING 

example in the maps of the United States Geological Survey. 
Maps are of such great importance in modern war, and are used 
in such great numbers, that the elaborate organization and plant 
necessary for the rapid printing of maps may be regarded as 
essential. On account of the additional facility in reading maps 
gained by the use of different colors, provision has also been made 
for printing even the battle maps in color. The use of color in 
military maps may therefore be regarded as the rule and not the 
exception. 

Relation of Conventional Signs to Map Scales. 

Since topographical data are recorded in the form of conven- 
tional signs, and the scale of the map determines the space avail- 
able in which to record the data concerning a given area, it is 
manifest that there is an intimate relation between the scale 
of the map and the use of conventional signs. 

This subject will be discussed further in Chapter VII in con- 
nection with the interpretation of map data in general. 

Familiarity with Conventional Signs. 

The conventional signs given on the following pages should be 
memorized so that any one of them will be readily recognized, and 
that any one of them can be reproduced from memory. There will 
be many others to be learned when foreign maps are encountered, 
but those here given will suffice as a foundation for work in map 
reading and topographical sketching. 

Exercise 1. Examine Plate A and see if all conventional signs used on it 
are recognized. 

Exercise 2. Examine similarly any other topographical maps available. 
The special Plattsburg map, scale 3 inches = 1 mile, published 
by the United States Geological Survey, is especially suitable 
for this purpose. 

Exercise 3. Ask someone to prepare a list of fifteen conventional signs 
from among those shown in Figs. 4-15 inclusive, and then 
draw all of these signs. 



MAP READING 
WORKS and STRUCTURES 



33 



Canal or Ditch 



Aqueduct or Waterpipe 



Aqueduct Tunnel 

Canal Lock (point up stream) 



Wagon Roads < 



Trail or Path 



Metaled 

Good - 

Poor or Private 

On small-scale maps 



Railroad of any kind ■> 

(or Single Track) 

Double Track 



I I I I I I I I I I I I 4— t 



Railroads I 



Juxtaposition of. 
Electric 



I I ) I I I I I | *T±t 



III I' 



t=t=l=t I I I | I I I I I I I I I I > I I I I I I II 

In Wagon Road or Street Steam Electric 



Tunnel 



| t i | | )r = = S sss=^fr- 



Railroad Station of any kind 



I I I — I I I H I 



Telegraph Line < 



Symbol (modified below) T TTTTTTT 
Along road 



Along road 
(small-scale maps) 

Along trail 



T — i — i — i — i — i — i — r 



-i~t— i — r"« — r— r— »— 

Electric Power Transmission Line , r „. 

Fig. 4. — Conventional Signs. 



34 MAP READING AND TOPOGRAPHICAL SKETCHING 
WORKS AND STRUCTURES 



Bridges 



General Symbol 



{ 



Drawbridges (on large-scale / 

charts leave channel open) V < i i i ; 

Truss (W, Wood; S, Steel) 

Foot 

Suspension 

Arch 

\ Pontoon 



Ferries 



(General Symbol - 

(or Wagon and Artillery) 



Fords \ Infantry and Gavalry 
\Cavalry 

Dam - - 





Fig. 5. — Conventional Signs. 



MAP READING 
WORKS AND STRUCTURES 



35 



Buildings in general 



J" 



D 



Ruins ... ihi 

Church - t or + 

Hospital _ .hos 

Schoolhouse tor.sti 

Post Office .po 

Telegraph Office i or .to 

Waterworks mWW 

Windmill & or * 



City, Town, or Village 




City, Town, or Village {generalized) 



City, Town, or Village 
(small-scale maps) 



Capital 

Gounty Seat 



1 



m 



Other Towns - - o 

Fig. 6. — Conventional Signs. 



V 



36 MAP READING AND TOPOGRAPHICAL SKETCHING 
WORKS AND STRUCTURES 



Cemetery L.^JoT-it 

Mine or Quarry of any kind (or open cut) ^ 

Prospect _ _ n> _ _ x 



Shaft 



Mine Tunnel 



(Opening A 

[Showing direction , ^ . > — 



Oil Wells o o o 



Oil Tanks (abbreviation OT) 



Goke Ovens 



Fence of any kind 
(or board fence) 



Stone .. 

Fences \ Worm 

Wire 



•• •• 




Hedge - ~ ass oo 9oo«« stvaso 

Fig. 7. — Conventional Signs. 



MAP READING 37 

BOUNDARIES, MARKS, and MONUMENTS 



National, State, or Province Line 
County Line 



Civil Township, District, 

Precinct, or Barrio 

Reservation Line _ 

Land-Grant Line 

City, Village, or Borough 
Cemetery, Small Park, etc. 



Township, Section, and Quarter Section 
Lines (any one for township line alone, any ' 
two for township and section lines 



Township and Section Corners Recovered— + -j._ i 

Boundary Monument .. <$— — — — 

Triangulation Station - a 

Bench mark _.. _ _ __ _ V 

1232 

U. S. Mineral Monument ▲ 

Fig. 3. — Conventional Signs. 



38 MAP READING AND TOPOGRAPHICAL SKETCHING 

RELIEF 



{Shown by contours, form lines, or shading as desired) 

Hill Shapes 



Contour System 




Depression Contours, if otherwise., 
ambiguous, hachured thus 



Rocky (or use contours) 



^f. ' I ' I ».| IT K 



Bluffs { 



\pther than rocky (or use contours) 




Sand Dunes 



Levee 



Fig. 9. — Conventional Signs. 



MAP READING 
LAND CLASSIFICATION 



39 



/Marsh in general (or Fresh Marsh) 



Salt.. 



Marsh < 



Wooded.. 



\Cypress Swamp. 






— /^ /^ 



*: 






Woods 0/ any Ar/no 7 (or as shown ie/ow) 



Solid w&sh of 
light green 



Woods of any kind (or Broad-Leaved Trees) 






O 



fe^ws-la 



Fig. 10. — Conventional Signs. 



40 MAP READING AND TOPOGRAPHICAL SKETCHING 
LAND CLASSIFICATION 



Pine (or Narrow-Leaved Trees) . 



Palm 



Palmetto. 



Mangrove 



Bamboo 







X 




* 


* 


-X- 


* •* 




-X 


H- 


* 


* 


* 




* 



Fig. 11. — Conventional Signs. 




MAP READING 
LAND CLASSIFICATION 



41 



Cactus 



Banana 



Orchard 



Grassland in general 



Tall Tropical Grass 



<f 


-< 


<> 


<f 


A 




A 


<> 




-o- 




n 



4- }f <¥ 



V 



jp 



4, 4- 



Q Q Q O 


(9- 


Q <2» © 


O 


O ® ® © 


O 



.MJ/fc 


* *l* 


nI< •'' 


a\I/.. "'• 


,\W/« 


,1/ 




Alfa 


vl, ..,1/,. 


v\l/. 



* 

i, 


i, 


X 



Fig. 12. — Conventional Signs, 



42 MAP READING AND TOPOGRAPHICAL SKETCHING 
LAND CLASSIFICATION 



Cultivated Fields in general 



Cotton 



Rice 



Sugar Cane 



Corn 







G 





o 


o 





O 














o 





/ <»'" 


1 




j<J//> 


1 


1 


-\l//. 




/\ ° 


i*. L. 




x \ 


u 1 




.\W/. 


-\\|A 


^ ' i 





-, 






A»/A. 






-v\l//. / 




.\U/. 



-f 1 


11 


<f 


* 


f 


«f« 


rf] 


+ 


rfl 


r|i 


J« 


+ 


«P 


t 


«f> 



«f T 


<f 


f 


1 1 


T 1> 


T 


T 


r 


r t 


? 


T 


T 



Fig. 13. — Conventional Signs. 



MAP READING 43 

HYDROGRAPHY, DANGERS OBSTRUCTIONS 



Shorelines i Surve »° d ~ 

I Unsurveyed . 



(Tidal Flats of any kind 
(or as shown below) 



Rocky Ledges 



Shores and 
Low-Water LJnes\ ad 



Gravel and Rocks 



Mud 




Fig. 14. — Conventional Signs. 



44 MAP READING AND TOPOGRAPHICAL SKETCHING 

DRAINAGE 



Streams in general - - /" 

Intermittent Streams f 



Lake or Pond in general 

(with or without tint, waterlining, etc.) 



Salt Pond (broken shoreline if intermittent)- 



Intermittent Lake or Pond 



Spring .Jf 

Fig. 15. — Conventional Signs. 




&S8& 




CHAPTER VI 
MAP READING— Continued 

Representation in Relief 

A map is of little value for military use unless it shows the 
form of the ground, since all military operations are affected by 
the character of the slopes, heights and positions of summits, etc. 
It is true that much may be inferred in regard to the general shape 
of the ground from the positions of the stream courses, especially 
if the general character of the country is known. But for military 
purposes it is necessary to have detailed knowledge of the ground 
forms, and the maps used must show them as accurately as 
possible. 

Methods of Representing Ground Forms. 

There are two general methods in use for representing ground 
forms : 

1. By hachures; and 

2. By contours. 

Present practice has crystallized in the use of contours to the 
exclusion of the older method of representation by hachures. 

Hachures, 

Hachuring has as its basis the representation of different 
slopes by means of shading strokes drawn in the direction in which 
water would flow on the ground at the point represented. The 
steepness of the slope is indicated by the length, weight, and spac- 
ing of the hachure lines. Gentle slopes are represented by long, 

45 



46 MAP READING AND TOPOGRAPHICAL SKETCHING 

light strokes, comparatively widely spaced. Very steep slopes 
are represented by short heavy strokes, closely spaced. Suitable 
variations between these limits are used to represent intermediate 
degrees of slope. Level spaces are left blank. 

For use in determining relative heights, the elevations above 
sea-level of points here and there on the map are given. These 
elevations are called " spot levels," and are stated in units cor- 
responding to the scale of the map. Thus in metric maps, the 
elevations are always expressed in meters; in inches-to-miles 
maps, they are expressed in feet. 

Fig. 16 shows an example of a hachured map with spot levels. 
It is a small section taken from one of the sheets of the 80 ,o'oo 
General Staff map of France. It is of especial interest because 
of its use in the European War. 

The defects of the method of representing slopes by hachures 
are: the absence of accurate representation; the very high degree 
of skill required to draw hachures acceptably; the time required 
to draw them; and the fact that on steep slopes the weight of the 
hachure shading almost precludes the representation of other 
information. The single counterbalancing advantage is that 
hachures give, in terms of light and shade, a sort of pictorial repre- 
sentation of the ground forms which is intelligible to an untrained 
eye and to the unimaginative mind. 

Similar in principle to hachuring is the method of stumping, in 
which the desired shading is accomplished by rubbing pencil dust 
into the paper. It is then fixed by the application of water. 
Stumping requires much less skill and time than hachuring, but 
it is difficult to reproduce in maps. 

Contours 

A contour is merely a line on the ground that marks all points 
of equal height above the mean level of the sea, or in other words 
simply a level line on the ground. 



MAP READING 



47 




Fig. 16.— Hachured Map. 



48 MAP READING AND TOPOGRAPHICAL SKETCHING 

Contours on the Ground. 

A physical conception of the line of a contour on the ground may- 
be gained by setting stakes at intervals on the same level by the 
use of a hand level or engineer's level. A piece of irregular ground 
is preferable for this purpose, as it gives a much better illustration 
of the meanderings of a contour. 

The illustration usually employed in books on topography to 
give a conception of contours on the ground is to conceive an 
island of irregular shape and with a variety of ground forms, 
and then to suppose that the water rises by successive equal 
stages. The shore lines at these various water levels would con- 
stitute a set of contours. 

It is well to gain the conception of a contour on the ground as 
the location of a level path. This conception of a path can be 
followed out on any piece of ground. By conceiving a series of 
paths at equal differences of level, a set of contours results. 

Contour Lines on the Map. 

For purposes of representation on a map, all topographical 
features are considered as having been projected by vertical 
lines upon a horizontal plane beneath them, just as if the ground 
represented had sunk to a level and each object had moved directly 
downward to this new level. They are shown to scale on the map 
in this relation to each other. 

For the purpose of representing contours on maps, the lines 
of the contours are considered to be similarly projected on a 
horizontal plane and drawn to scale in their proper positions 
relative to the other topographical features. 

The lines on the map which represent the contours on the 
ground are also called contours. In the following discussion it 
will be necessary to distinguish between the contours on the 
ground and the contours on the map by continual repetition of 
these terms. 



MAP READING 49 

Datum Level. 

For convenience, all elevations on topographical maps must 
refer to some datum, and the one regularly used is mean sea- 
level. The elevations shown upon maps resulting from formal 
surveys' always represent vertical heights above mean sea-level. 
In small local surveys and for purposes of instruction, if the 
elevations above mean sea-level are not locally known, or can- 
not be referred to conveniently, the datum level may be assumed 
at some level below the lowest point in the area to be repre- 
sented on the map. This is done by assuming the elevation of 
some point in the area to be 100 feet, or any convenient value, 
such that no point in the area will be below zero level. The 
elevations of the contours on the ground are marked upon the 
corresponding contours on the map. 

Vertical Interval between Contours. 

Although ground forms might be shown on a map by con- 
tours representing random differences of level, convenience re- 
quires that successive contours shall represent uniform differences 
of level. This difference of level between contours is called the 
vertical interval, abbreviated V.I. 

The actual elevations represented by the contours are always 
expressed in meters on metric scale maps, and in feet on inches- 
to-miles maps. Although the V.I. may be 1 foot or 2 feet, or 
1 meter or 2 meters for very large scale maps, the V.I. is usually 
5 units or some multiple of 5. The usual contour intervals 
encountered in our units are 5, 10, 20, 50, and 100 feet, and in 
the metric units 1, 2, 5, 10, etc., meters. 

The reason for the use of a uniform V.I. is that it is desirable 
that the contours on the map shall show the slopes as well as 
the form of the ground. With a uniform V.I., the map con- 
tours, which represent the horizontal projections of the ground 
contours, will be close together where the ground is steep, and 
far apart on gentle slopes. Thus the map spacing of the con- 



50 MAP READING AND TOPOGRAPHICAL SKETCHING 

tours will reflect accurately the slope of the ground. In fact, 
in rugged country, the contours give an effect of shading the steeper 
slopes similar to that of hachuring. 

Methods of Stating Slopes. 

For military purposes, it is customary to state slopes in 
degrees, but they may also be expressed as percentages, or as 
gradients.. The degree of a slope is the angle in degrees which 
it makes with the horizontal. It is measured directly by the 
reading of the vertical circle of a transit, or by the reading of a 
clinometer. 

The percentage of a slope is given by the number of units 
rise or fall in 100 units horizontal distance. Thus a 2 per cent 
slope means that there is a vertical rise or fall of 2 feet in 100 
feet horizontal distance, or of 2 meters in 100 meters, etc. 

The gradient of a slope is the ratio of the vertical rise or fall 
to the horizontal distance in which this change occurs, both 
expressed in the same units. Thus a rise or fall of 1 foot ver- 
tically in 12 feet horizontal distance may be called a slope of iV, 
also referred to as a slope of 1 on 12. 

Slopes are referred to as + or positive if the slope is upward 
from the point of observation, and — or negative if the slope 
is downward. 

The degree, percentage, or gradient of a slope may be cal- 
culated from the contours on a map. The contour interval 
gives the rise or fall in the horizontal distance shown on the 
map between contours, and the ratio of these distances may be 
converted by calculation into degrees, percentage, or gradient. 

Since each calculation would thus have to be made inde- 
pendently, it is much more convenient to construct a graphical 
scale for reading slopes. 

Relation between Slope and Distance between Contours. 

For the sake of convenience in constructing slope scales, it 
is assumed that the tangents of angles are directly proportional 



MAP READING 



51 



to the angles themselves. This is not strictly true, but the 
error in the assumption is very small, as will be seen from the 
fact that the natural tangent of 1 degree is .01746, of 10 degrees 
is .17033, of 15 degrees is .20795, and of 20 degrees is .36397. 
From a comparison of these values, it will be seen that the error 
in the assumption increases with the degree of slope, but that 
it is still small even for 20 degrees, which is steeper than the 
slopes usually encountered in all but the most rugged country. 

Under the assumption that the tangents are directly pro- 
portional to the slopes, it results that the distance between con- 
tours of equal intervals is inversely proportional to the slope 




Fig. 17. — Map Distance between Contours. 



represented. Thus the contours representing a 2-degree slope 
would be half as far apart as those representing a 1-degree slope, 
as will be seen by reference to Fig. 17. 

In this figure the distance AB represents the vertical interval 
between contours, and the distance OA is the horizontal distance 
between them. If the distance BC be set off equal to the dis- 
tance AB, the assumption that the slope in degrees of the line 
OC is double that of the line OB is very nearly true, and can 
properly be made with reference to lines no more accurately 
drawn than are contours, even on the best of maps. 

From the figure it will be seen that the point D is at the 
same elevation as the point B, and that the distance OE, which 



52 MAP READING AND TOPOGRAPHICAL SKETCHING 

is the distance between contours on the slope OC, is one-half 
the distance between contours on the slope OB. 

Under the same assumption that tangents are directly pro- 
portional to angles, a similar relation could be established in the 
case of slopes expressed in percentages or as gradients. Thus 
the contours are twice as far apart on a 5 per cent slope as on 
a 10 per cent slope, and twice as far apart on a ^V slope as on a 
iV slope. 

Relation between V.I. and Map Distance between Contours. 

The actual horizontal distance between contours on a given 
slope is directly proportional to the V.I. Thus on a 5-degree 
slope, contours of 20 feet V.I. are twice as far apart as are con- 
tours of 10 feet V.I., and the map distance between contours 
bears, of course, the same relation. If, however, the 20-foot 
contours are drawn on a map-scale half as large as that on which 
the 10-foot contours are drawn, the map distances between 
contours representing the same slope will be the same on the 
two maps. 

From the above, it is apparent that such a ratio may be 
maintained between the V.I., and the scale of the map that the 
same map distance between contours will always represent the 
same degree of slope. This is very satisfactory in theory, and 
would be very convenient in practice were it not for the fact 
that the V.I. must be selected with reference to the character of 
the country to be represented. Areas where the differences of 
elevations between summits and valleys are small require a small 
V.I. for adequate representation. On the other hand, in moun- 
tainous country the use of a small V.I. would result in such a 
number of contours that the map would be obscured by them. 

The primary purpose of a map is to represent the ground. 
To do so, the V.I. must be selected to accord with the character 
of the country, and not to fit some theory based upon an easy 
and mechanical reading of slopes. 



MAP READING 53 

Construction of Slope Scales. 

In the construction of slope scales, two factors enter: 

1. The vertical interval between contours; and 

2. The scale of the map. 

A line having a slope of 1 degree rises 1 foot vertically in a 
horizontal distance of 57.3 feet, or 10 feet in 573 feet. Therefore 
the ground distance between contours of 10 feet V.I. on a slope 
of 1 degree is 573 feet. Under the assumption that tangents are 
directly proportional to the angles themselves, the ground distance 
between contours of 10 feet V.I. on any other degree of slope 
would be 573 feet divided by the degree of slope. Thus the 
ground distance on a 2-degree slope would be 286.5 feet; on a 
3-degree slope, 191 feet; on a 5-degree slope 114.6 feet; and 
similarly for all other degrees of slope. 

The ground distance between contours is directly proportional 
to the V.I. For a 5-foot V.I., the distance would be half those 
given above, for a 20-foot V.I. double the distances, and similarly 
for other intervals. 

To construct a slope scale, it is only necessary to calculate the 
map distances corresponding to the V.I. and map scale desired, 
and then set off these distances on a line in some form convenient 
for use. 

Two forms of slope scale are required: 

(a) One showing the map distance for each degree of slope re- 
peated only once, for use in measuring slopes on maps; and 

(b) One showing the map distance for each slope repeated a num- 
ber of times, for use in plotting contours in map making. 

The two forms are shown to scale in the figures illustrating 
the examples following. 

Example 1. Let it be required to construct a slope scale for 
use in reading slopes. Scale of map, 6 inches = 1 mile. V.I. = 
10 feet. 



54 MAP READING AND TOPOGRAPHICAL SKETCHING 

Solution. The ground distance between contours for 1-degree 
slope for V.I. = 10 feet is 573 feet, and for all slopes the distance is 
inversely proportional to the degree of slope. 

The R.F. corresponding to 6 inches = 1 mile is 63 3 60 inch = 
10 g 60 . 573 feet = 6,876 inches. The map distance corre- 
sponding to 573 feet is 1 of 5 7 6 6 =.651 inch, which is the map 
distance between contours of 10-feet V.I. on a 1-degree slope. 
For a 2-degree slope, the map distance is ^^^ = .325 inch; 
for 3 degrees, ^f 1 = .217 inch; etc. Set off the calculated 
lengths of the map distances corresponding to the various degrees 
of slope successively on a line, and mark the divisions with the 
corresponding slopes, as shown in Fig. 18. This scale is applied 
to the map to determine the scale division most nearly equal to 
the perpendicular distance between contours, and the slope of the 
ground is given by the marking of the division. 



1° 2° 3° 4° 5°67 e OT) 

i i i i i i i i i i 



Fig. 18. — Slope Scale for Map Reading. 

Example 2. Let it be required to construct a slope scale 
for plotting contour intervals. Scale of map 10 ^ . V.I. = 5 
meters. 

Solution. The V.I. being 5 meters,' the map distance between 
contours on a 1-degree slope will be 57.3X5 = 286.5 meters. It 
is to be noted that in the metric system, the map distance could 
be taken directly from the reading scale of kilometers. To con- 
struct the slope scale from a scale of inches, it is necessary to 
reduce to our units. The distance 286.5 meters reduced to inches 
is 286.5X39.37=11,279.5 inches. The map distance correspond- 
ing to this ground distance is Vo.Vo 'o 5 = 1-128 inches. Correspond- 
ing fractional parts of this distance give the map distances for the 
other degrees of slope. 



MAP READING 



55 



Since it is desired to repeat each contour interval a number of 
times on the scale, it is best to construct five separate scales, hav- 
ing as the smallest divisions 12, 10, 9, 8 and 7 degrees respectively. 
Two divisions on each scale will represent the map distance be- 
tween contours for one-half the degree of slope. Hence, by suitable 
marking the 12-degree scale will answer for 12, 6, 3, and 1| degrees; 
the 10-degree scale for 10, 5, and 2| degrees; the 9-degree scale for 
9 and 4 \ degrees; the 8-degree scale for 8, 4, 2, 1, and \ degrees; 
and the 7-degree scale for 7 and 3| degrees. These five scales 
cover all slopes up to 5 degrees by intervals of \ degree, and from 
5 up to 12 degrees by intervals of 1 degree (except 11 degrees). 
These scales are shown in Fig. 19. They are printed on the 



i I I I I I i 

4 I ' 



1 1 1 1 1 1 1 



6 I 



Mil 
9°L I 

4 



ty I ' I ' I ' 



50 k 



Fig. 19. — Slope Scale for Map Making. 



Mill 



sketching board and also on the edges of the alidade of the stand- 
ard sketching equipment in use by the United States Army. 

Scales of this form may also be used for reading slopes on maps, 
but they are not nearly as convenient for this purpose as the form 
shown in Fig. 18. 

It is to be noted that a slope scale can be used for different 
map scales and vertical intervals than those for which it was 
constructed, provided the new scale and V.I. have certain rela- 
tions to the old. Thus a slope scale for a map of scale 6 inches = 1 
mile and V. I. = 10 feet may also be used for the same map scale 
and V. I. = 5 feet by halving all the intervals; for a map scale 
3 inches =1 mile and V.I. = 20 feet without change; or for a scale 
of 3 inches = 1 mile and V.I. = 10 feet by halving the intervals. 



56 MAP READING AND TOPOGRAPHICAL SKETCHING 

The use of slope scales in map making will be discussed in a 
later chapter under the subject of topographical sketching. 



Problem 1. Construct a slope scale for use with map of scale - — and 

1 ^ 20.000 

V. I. =5 meters. 
Problem 2. Construct a slope scale for use with the United States Geo- 



-, V. I. =50 feet. 



logical Survey maps, scale — 

6!b.uvu 

Problem. 3. Construct a slope scale for use with Plate A, scale — 1 — 

1 1 0,56 o - 

V. I. = 10 feet. Upon Plate A, identify slopes of 1, 2, and 
3 degrees. 



CHAPTER VII 

MAP READING— Continued 

Contours — Continued 
Forms of Contours. 

Although the forms of contours can be studied much better 
on the ground, a certain familiarity with the subject can be 
acquired by indoor study, illustrated by clay or plaster models 
or by the sand box. 

It is desirable to revert to the conception of a contour as 
a level path upon the ground, and to consider the process of 
following a contour on the ground as that of selecting the loca- 
tion of such a level path. 

Valley Contours. Let us consider ourselves as being upon 
the sloping side of a narrow valley, and carry out in imagination 
this process of following a contour up the valley. The surface 
of the stream itself rises toward its source, therefore there will 
be some point on it which is at the same level as the assumed 
starting point. If we start up the valley, keeping on a level 
path, we will gradually come nearer the stream and actually 
reach it at the point level with the starting point. After crossing 
the stream we must turn sharply down and follow on the oppo- 
site side of the valley a course similar to that followed in going 
up-stream, gradually getting farther from the stream as we pro- 
ceed. The course followed marks a contour and its map repre- 
sentation would be of the form shown in (a), Fig. 20. The 
arrowhead indicates the direction in which the stream flows. 

57 



58 MAP READING AND TOPOGRAPHICAL SKETCHING 

* If a small tributary stream had been encountered before the 
contour crossed the main stream, the contour would have turned 
up the valley of the tributary, crossed it and returned on the 
other side to the main valley, when it would again have turned 
up the main valley as before. The resulting contour form is 
that shown in (6), Fig. 20. Additional tributaries would have 
resulted in the contour taking the general forms shown in (c) 
and (d), Fig. 20. 

The forms of contours shown should be remembered as typical 
forms of valley contours. The width of the opening of the V 
of the contours as compared with the total distance up-stream 
to the point of the V will vary with the characteristics of the 







0>) (c) 

Fig. 20. — Typical Valley Contours. 

valley. Thus in a narrow valley with steep sides and no level 
bottom, the V of the contours will be long and narrow, whereas 
in a wide valley with gentle slopes, the V shape will be usually 
more open. The actual forms for these different cases should 
be studied and drawn on the ground. It will always be a sur- 
prise to the beginner in topographical mapping to find how far 
up-stream the loops of the valley contours extend. 

If the tributary valleys had been small in extent, insufficient 
to collect enough water to form a running stream, the presence 
of these small tributary valleys would have been indicated by 
the form of contours shown in Fig. 21. 

Spur and Ridge Contours. If we consider the spurs and 
ridges between tributaries of the same stream, some of them 
will be found to be narrow and sharp, marked by a narrow U-shaped 



MAP READING 



59 



form of contour, as shown in (a), Fig. 22. Varying degrees of 
the sharpness of the ridges wilt be marked by different forms of 
the U-shaped contours, as shown in (6) and (c), Fig. 22. The 
forms of these spurs and ridges are determined by erosion, and 
they reflect the plan of the streams which have carried out this 



(a) (b) (c) (c/) 

Fig. 21. — Dry Tributary Valleys. 

work. In illustrating these typical forms of contours, the student 
is reminded that they are to serve him only temporarily until 
he is prepared to work them out for himself on the ground. 

Summit Contours. Turning now to the high ground, there 
will be found along the ridges between the branches of the streams, 






Fig. 22. — Spur and Ridge Contours. 



and particularly along the main ridges separating the headwaters 
of different streams, occasional hills higher than any of the sur- 
rounding ground. A contour traced near the summit of such 
a hill would not wander off indefinitely into the surrounding 



60 MAP READING AND TOPOGRAPHICAL SKETCHING 

country, but would pass completely around the summit of the 
hill and close upon itself. If the summit of the hill were round, 
the contour would be circular in form; if oblong, the contour 
would be oval; and in nature such summits will be found in 
an infinite variety of forms, with corresponding variety in the 
shapes of the contours representing them. 

As we go downward from the summit, the contours show the 
irregular beginnings that develop into the ridges, spurs, and 
valleys of the lower ground. 

Col or Saddle Contours. One ground form which should be 
mentioned and studied is the col or saddle. This form usually 
results when a ridge separating two streams is indented by nearly 
opposite tributaries of the two streams, also in similar cases along 
the ridge forming the high ground or watershed separating two 
different drainage basins. The latter form is usually called 
a gap. » 

In this type form we have the situation shown in (a), Fig. 
23, where the point marked X is a low point on the ridge line. 
From X it would be necessary to go up hill to get to the top 




£0 



SO 



g' 60 

J h K 60 

(C) 
Fig. 23— Col or Saddle. 




of the ridge in either direction, whereas the ground would fall 
toward the stream on each side. The map contours representing 



MAP READING 



61 



the col or saddle are shown in (b), Fig. 23. In this form the 
highest pair of valley contours, e and /, in (c), Fig. 23, are always 
a contour interval lower than the lowest pair of ridge contours, 
g and h, 

Representation of Slopes by Contours. 

Contours show by their trace the form of the ground, and 
by their spacing not only the declivity but also the character 
of the slopes. The latter is very important from a military 
standpoint. 

Uniform* Slopes. From any point on a uniform slope the 
whole of the slope can be seen, and the slope can be swept either 
up or down by a grazing fire. 






? if) So' 

'(a) 0) 

Fig. 24. — Concave and Convex Slopes. 



A uniform slope is indicated by a uniform spacing of con- 
tours, — steep slopes by close and gentle slopes by wide spacing. 

Concave Slopes. A concave slope is one which grows steeper 
as it rises. From any point on such a slope the whole of the 
slope can be seen, but fire aimed at any point on the slope will 
pass too high above the other portions of the slope to be effective. 



62 MAP READING AND TOPOGRAPHICAL SKETCHING 

The contours representing a concave slope diminish in spacing 
toward the top of the slope as shown in (a), Fig. 24. 

Convex Slopes. A convex slope grows more gentle as it rises. 
From any point on a convex slope the view and field of fire are 
limited. 

The contours representing a convex slope increase in spacing 
toward the top of the slope, as shown in (b), Fig. 24. 

Exercise 1. That portion of Plate A lying north of the road past the water- 
works should now be examined to identify valley, spur, 
summit, and saddle forms, also uniform, convex, and con- 
cave slopes. 

Exercise 2. There is an error in numbering one of the contours in Plate A. 
Find the error and correct it. 

Plate A is a map of an area of comparatively small differences 
of elevation and gentle slopes. Any other maps available should 
be examined to identify the ground forms mentioned. 

Models and Sand Box. 

In gaining a conception of the type ground forms, models 
made of plaster of Paris or of some of the market preparations 
of modeller's clay, or the sand box may be used. The models of 
plaster or clay may be worked out as careful reproductions of the 
ground as represented by a map. The contours may be marked 
upon the models, and in a general way they will be found of assist- 
ance in aiding the student's conception of the contoured repre- 
sentation of ground forms. 

Of practically equal service is the shallow box of moist sand, 
and this can be made available for any class at trifling cost. The 
moist sand is modelled into forms for illustration, and the con- 
tours can be indicated by lines marked on the sand. Ground 
forms may be modelled in the sand box to be used as the basis of 
contoured representation on a reduced scale. 

The difficulties with the use of models or the sand box for. 
instruction in topography are that in them the vertical scale 



MAP READING 63 

must be greatly exaggerated, and that the models reproduce the 
map and not the ground from which the map was made. The 
models, therefore, give no conception of the errors and generaliza- 
tions of the map itself as a representation of the ground, nor do 
they give any conception of the difficulties that confront the 
topographer on the ground. 

The reason for using an exaggerated vertical scale in construct- 
ing models of ground forms may be seen by assuming that Plate 
A is to be reproduced as a relief model. The scale of this plate is 
6 inches = 1 mile, so it would be reproduced in full size as a plan 
of the area. The total difference of elevation or relief is 150 feet. 
The map distance corresponding to 150 feet on a scale of 10 j 60 
is ~n^jg\) 2 = '17 of an inch. In other words, if a relief model the 
size of Plate A were constructed to show this ground, the total 
vertical height necessary to show the relief to scale is less than i 
of an inch. Yet the ground itself would give the impression of 
being much rougher than would the scale model. To overcome 
this difficulty, it is customary to exaggerate the vertical scale of 
the model to 5 or 10 times the horizontal scale. 

This exaggeration of the vertical scale is necessary, but the 
difficulties of model representations must be borne in mind in 
order to avoid false conceptions. 

Contour Problems. 

Further familiarity with the type forms of contours can be 
gained by drawing the contours of an area, having given the 
stream courses and the elevation of points where the slope of the 
ground changes. This class of problems will be taken up in con- 
nection with the subject of topographical sketching in Chapter 
XL 

Construction of Profiles. 

Problems in the construction of profiles along lines drawn on a 
map give excellent practice in gaining familiarity with the inter- 
pretation of contours. The same objection as in models will, 



64 MAP READING AND TOPOGRAPHICAL SKETCHING 

however, be encountered with respect to the necessary exaggera- 
tion of the vertical scale. 

Profile or cross-section paper is convenient for the construction 
of profiles, but if not at hand plain paper can be used, or the profile 
can be constructed directly on the map itself. 

To construct a profile, select on the map the line of which 
the profile is to be drawn. This line may be either straight or 
meandering. Select a line of the profile or cross-section paper to 
represent datum level, or draw such a line on the plain paper. 
On the edge of a piece of paper take off from the map all points 
along the profile line where contours and streams cross it, and 
transfer these points to the line on the paper on which the profile 
is to be drawn. On perpendiculars at these points set off, to the 
vertical scale selected, distances corresponding to the heights repre- 
sented by the contours, and mark also the positions of the crests 
and stream crossings. Connect these points by a continuous 
line without any abrupt breaks. This line will represent a profile 
of the ground along the line selected, as shown in Fig. 25. 




Fig. 25. — Visibility of Points and Areas. 

The high points where the profile line crosses the crests of 
spurs or ridges and the low points where it crosses the streams 
will have to be determined in elevation by estimation, using the 
theory that the slope is uniform between contours. Let us assume 
that a profile is to be constructed on the 16-minute line of Plate A. 
Just north of the waterworks road the profile line crosses the spur 
between Indian Creek and its tributary. At the highest point 



MAP READING 65 

of the profile line just west of the oval 100-foot summit contour, 
the profile line gains about .6 of the interval between the 70 and 
80-foot contours. The highest point of the profile line is therefore 
76 feet where it approaches nearest the 80-foot contour. Just 
south of this road, the profile line crosses the tributary of Indian 
Creek. The level at the tributary would have to be determined 
by first determining the height of Indian Creek at the bridge, 
which is about ^o of the distance between the crossing of the 
40-foot contour northwest of the bridge, and the crossing of the 
50-foot contour southeast of the bridge at the edge of the woods. 
The elevation of Indian Creek at the bridge is therefore 41 feet. 
The profile line crosses the tributary about \ of the distance 
between the bridge and the 50-foot contour crossing, or at an 
elevation of about 43 \ feet. 

Problem 1. Construct a profile along the pipe line from the corner of 
the waterworks to the reservoir. Plate A. 

Problem 2. Construct a profile of the road from Red River to the reser- 
voir. Plate A. (Note. — The fraction written at the bridge 
over Indian Creek, 1 90 x 1 5 , means that the roadway of the 
bridge is 190 feet long and 15 feet wide, and that the road- 
way is 20 feet above the level of the water.) 

Problem 3. Is the data for the bridge near the northeast corner of 
Plate A correct? 

Visibility. 

Another excellent exercise for gaining familiarity with the inter- 
pretation of contou's and slopes is the problem of determining 
whether one point is or is not visible from another point on the 
map. 

It is necessary to warn the student that, due to the inherent 
inaccuracy of contour representation, the evidence drawn from 
the map with regard to the visibility of points is not absolutely 
conclusive concerning the actual visibility or otherwise of the 
points on the ground, but this does not detract from the value of 



66 MAP READING AND TOPOGRAPHICAL SKETCHING 

visibility problems as an exercise in the interpretation of contours 
on the map. 

It will be seen from an examination of the profile drawn under 
Problem 1 above that the profile will serve to show whether any 
ground form along the line shuts out the view between two selected 
points on the profile. This is true notwithstanding the exaggera- 
tion of the vertical scale, because the slope of the line representing 
the visual ray is exaggerated in the same proportion as all other 
vertical dimensions. If a straight line drawn between two points 
on the profile lies above all intermediate points of the profile, 
then each point is visible from the other. 

The point of view may not be the surface of the ground, but 
may be the height of the eye of an observer standing, or the 
observer may be in a tree or other elevated observation point 
such as a tower or captive balloon. In these cases the height of 
the observation point above the point in the profile must be deter- 
mined and plotted. 

If in a profile a line be drawn from any observation point 
tangent to a hill, the point of tangency will show the limit of 
visible points on the hill itself, and the line of sight prolonged 
will show how much of the succeeding valley is screened from view 
and how much of the surface lying beyond can be seen. This is 
illustrated in Fig. 25 in which is the point of observation. All 
ground is visible to the point A, where the ground dips and cannot 
be seen until the point B is reached. The ground from B to C is 
visible, but all ground from C to D is concealed by the hill C. 
These points A, B, C, and D can be transferred from the profile 
to the map. Successive profiles radiating from will serve to 
determine a succession of points A, which when joined on the map 
will form a line of screen, and with points B will serve to outline 
an area invisible from 0. Similarly, the invisible area of which 
C D is a part can be determined. 

It is not necessary to construct the full profile in order to 
solve problems of visibility. The elevation of the point of obser- 



MAP READING 67 

vation is given by the conditions of the problem. An examina- 
tion of the ground forms and of the spacing of the contours will 
serve to disclose the probable points of screen. A construction 
of the profile at these points may be necessary to determine the 
exact location and elevation of the screening point. Having 
established the elevation of two points on the line of sight, the 
elevations corresponding to contours can be marked off on it, 
and these elevations compared with the elevations shown on 
the contours on the map. So long as the elevations along the 
line of sight are greater than the elevations of the contours of 
the ground, the ground is invisible. When a point is reached 
where the opposite is true, the ground becomes visible. The 
point where the line of sight strikes the surface can usually be 
determined approximately by inspection. Since the solution 
is not exact anyhow, no great degree of precision is necessary 
in determining where the line of sight strikes the ground. 




Fig. 26. — Line of Screen on Spur. 

In determining the line of screen on a spur, it may only be 
necessary to draw from the observation point lines tangent to 
the contours of the spur, and then connect the points of tan- 
gency, as in Fig. 26. To permit the use of this method, the ob- 
servation point must be neither too close to, nor much higher 
or lower than the spur, otherwise the solution may not be true. 

Thus in Fig. 26, if the line OB, tangent to the 80-foot con- 
tour, be regarded as a profile line, the point B will be the highest 
point in this profile and hence the screening point, unless the 



68 MAP READING AND TOPOGRAPHICAL SKETCHING 

point be either close to and higher than B, in which case the 
point B' may be visible, or if the point be close to and much 
lower than B, the point B ' may screen the point B from view. 
In either of these cases it is only necessary to compare the slope 
of the line OB with either that of BB' or BB" . 

When models of the ground are available, the solution of 
visibility problems is simplified by the use of a small flash-light. 
This light. is placed at the location of the observation point, and 
such areas of the model as are in shadow are screened from 
view from the observation point. 

The question of visibility may be solved from either our own 
point of view to determine the points within the enemy lines 
which are hidden from our view, or from the enemy point of 
view to ascertain the areas wherein our own movements and 
works are hidden from him. Of particular importance are ques- 
tions of the points visible from the captive balloons, which are 
now much used for observation purposes. 

Visibility Problems. 

Visibility problems fall into five general classes: 

(a) Is the point A visible from the point B? Solve for visibility as 
already explained. 

(b) How high would an observation tower (or captive balloon) 
at the point A have to be in order to see the point B? 

From the point B find the screening point in the direction of J.. 
From the height of B and of the screening point, find the ele- 
vation of the line of sight from B at A . The difference be- 
tween this elevation and that of the ground at A is the required 
height of the observation point. 

(c) How high above ground would an observation balloon have to 
be at the point A in order to see all of the ground within a 
specified area? 

Examine the specified area to find the steepest slope (i.e., 
closest contour spacing) facing away from A . The contour 



MAP READING 69 

spacing must be measured along lines radiating from A. This 
contour spacing and the V.I. of the map determine the maximum 
slope the line of sight from A must have in order to see all of 
the area. From these data find the height of the point of sight 
at A. 

If the area is large, several slopes may have to be investigated, 
because a slightly smaller slope at the farther edge of the area 
might require a higher observation point. In this case use 
the highest elevation determined. 

If the height of the observation point above the ground is re- 
quired, subtract the elevation of the ground at A. 

4 (d) What areas within certain limits are not visible from the point A? 
Solve as already explained for visibility of areas. 

(e) Observation balloons observe regularly from an altitude of 5,000 
feet above datum at the points A and B. Is there any point 
near C where a building 30 feet high can be built which cannot 
be seen from either of the observation balloons? 
Find the screened areas near C for each of the balloons inde- 
pendently. Investigate the points where these screened areas 
overlap. 

Problem 1. Plate A. Are the waterworks visible from the 100-foot 
hill east of 16 minutes on the southern border of the map? 

Problem 2. Plate A. How high would a flag pole at the Marion Junction 
station have to be to be seen from the belfry of the church 
in Weston, 30 feet above the ground? 

Problem 3. Plate A. How high would an observation balloon at Marion 
Junction have to be to see all of the area included between 
the waterworks and reservoir roads? 

Problem 4. Plate A. What parts of the road east from the waterworks 
are visible to a man standing on top of the 100-foot hill 
south of the reservoir? 

Problem 5. Using any map available, solve a problem similar to (e) 
above. The area shown by Plate A is insufficient for such 
a problem. 



70 MAP READING AND TOPOGRAPHICAL SKETCHING 

Although visibility problems are valuable exercises in the 
study of contours on a map, it is well to caution the student 
once more against placing too much confidence in the results of 
the solutions of these problems as applied to the ground. In- 
accuracies in map locations, inaccuracies in the contours, and 
the screening effect of trees and other vegetation introduce 
factors of uncertain effect. 

Problem 1 may be solved in good faith, but in practice there 
is tall tropical grass on the 100-foot hill and bamboo on the 
screening point. In Problem 3 a little error of the topographer 
or a little slip of the draughtsman's pen would affect the answer 
by several hundred feet. The true test of visibility is on the 
ground itself. 



CHAPTER VIII 

MAP READING— Continued 

Map Reference 

Designation of Points on Maps. 

In the problems at the end of Chapter VII it was necessary 
to identify points on Plate A by descriptions of considerable 
length. Thus one point was referred to as " the 100-foot hill east 
of 16 minutes on the southern border of the map." For random 
points on the map. it is impossible to frame a description which 
will identify them accurately. 

In the military use of maps, it is essential to be able to identify 
unmistakably, and to describe briefly and accurately in an order 
or message, any point on any map to which it is desired to refer. 

Map Sheets. 

General topographical maps issued in time of peace usually 
consist of a series of practically rectangular sheets, all on the 
same scale. Areas of especial importance may be issued on 
larger scales. These map sheets are usually identified by the 
name of the principal town within the limits of the sheet. 

If the military forces have occasion to prepare their own 
maps, the varied uses for which they are intended will require 
that maps of several different scales be issued. Thus there 
may be a general map on a scale of 40 * 00 with issues of special 
areas on 20 * 00 > 10 * 00 , etc. If the successive scales of the 
map sheets are in this proportion, then four sheets of the 20 * 00 
map may be made to cover the same limits as one sheet of the 

71 



72 MAP READING AND TOPOGRAPHICAL SKETCHING 

An * , four sheets of the . n * the same limit as one sheet 



°f th e 20,000 ' e ^ c -j an d a ll ma P sheets will be of the same actual 
size. 

Map Reference. 

For military purposes, it is necessary to identify a point as 
being on a particular sheet of a given series of maps, and then 
as being at a particular point on that particular sheet. Map 
reference is the term applied to the system by which this is 
accomplished. 

Different systems of map reference are in use. It is not 
desirable to describe any of them in full detail, because the 
explanation would be complicated, and any system of map 
reference can be learned in a short time with maps exemplifying 
the system at hand. It will suffice here to describe the general 
principles governing them all. 

Reference Lines. It is first necessary to assume a set of coor- 
dinate axes to which all points on every sheet of the whole series 
of maps can be referred. Reference lines parallel to the coor- 
dinate axes and at convenient intervals, such as 1,000 yards 
or 1,000 meters, are overprinted on all map sheets of the whole 
series. Such a set of reference lines is termed a grid, and divides 
the map "sheets into squares. 

In order to prevent confusion, it is desirable that the grid 
shall be continuous for the entire system of maps, that the map 
sheets shall be square or rectangular in form, and that the boundary 
lines of all map sheets shall correspond to lines of the grid system. 

For purposes of map reference, the grid may be used in two 
different ways: (a) the grid lines may be numbered consecutively 
from some origin and all points given by coordinates from the 
origin; or (b) the map sheets may be laid out to correspond to 
lines of the grid and points on each map sheet designated by 
their coordinates on that sheet. 

The first system, which may be called the coordinate system, 



MAP READING 73 

is very simple in its application. All sheets of whatever scale 
have the grid overprinted with each line bearing its number 
from the origin. The points are designated by their coordinates 
from the origin and may be found readily on any map of any 
scale. 

The second system, which may be called the sheet system, re- 
quires that the reference system shall be worked out on the sheets 
of some selected scale, and a suitable reference grid overprinted 
on the map sheets of this scale. The larger scale subdivisions 
of these standard sheets must each bear its corresponding part 
of the grid carrying the same designations as on the standard 
sheet, in order that the same point may have the same reference 
designation on the map sheets of different scales. This system 
requires special means for identifying the map sheets of the 
various scales. 

Designation of Map Sheets. The sheets of the general map 
may be either numbered or named. The name usually assigned 
to a sheet is that of the principal town on the sheet. Numbers 
are assigned serially in some regular order. The numbers or 
names of the general map sheets are usually shown on a key 
map. 

We may assume that one of the general map sheets, scale 
40 l 00 , has been designated as sheet 12. The four corresponding- 
sheets of the 20 p 00 map may then be designated 12 NE, 12 SE, 
12 SW, and 12 NW. The four 10 l 00 sheets corresponding to 
12 NW may be designated 12 NWa, 12 NWb, 12 NWc, and 12 
NWd. If the area shown by sheet 12 NWd is to be represented 
on a scale of 5i q 00 , the four sheets can be designated 12 NWdl, 
12 NWd2, 12 NWd3, 12 NWd4. All sheets covering portions 
of sheet 12 are thus identified with it by their designations. 

Sheet System of Reference. Upon whatever large-scale sub- 
divisions of sheet 12 a point may be shown, the reference number 
for it must be the same. This is accomplished by subdividing 
sheet 12 into a number of lettered squares, as A, B, etc. These 



74 MAP READING AND TOPOGRAPHICAL SKETCHING 

lettered squares are then subdivided into numbered squares, 
as Al, A2, etc. Squares of this designation should be 1,000 
yards or 1,000 meters on a side. They may again be subdivided 
into four squares each 500 meters or yards on a side, designated 
by additional small letters, as A5a, A5b, Abe, and A5d. 

The large-scale subdivisions of sheet 12 must show the appro- 
priate portion of this grid with its squares properly designated, 
so that the square designated M25d on sheet 12 shall be pos- 
sible of identification as the same square on the 20 ,o 00 , 10i ^ 00 , 
etc., sheets which show it. The general scheme of such a sys- 
tem is shown in Fig. 27. 

Location of Points in Grid Squares. The means by which the 
grid squares are identified have been pointed out. Points in the 
grid squares are located by their coordinates in the square, as 
shown in Fig. 28. Suppose the figure to represent grid square 
^46 of sheet 12, this square being 1,000 yards on a side. The 
lines representing the subdivisions into the a, b, c, and d squares 
may be shown broken, the subdivisions into tenths on the sides 
of these squares indicated by marks, giving the necessary means 
of subdividing into squares, as shown in square 6a. The coor- 
dinates of any point in square 6a may be given by its dis- 
tance from the lower left (or right) hand corner of the square, 
first the horizontal distance and then the vertical, as 6a87 for 
the small circle marked. For more accurate identification the 
coordinates in the small square may be estimated, as 6a2218 
for the point within the small circle. In this case the 22 is the 
horizontal coordinate, and 18 the vertical coordinate. In this 
system the sheet coordinate designation must be added to the 
sheet number, and the full description of the last point is 
12A6a2218. 

In the continuous coordinate system, the coordinates of a 
point in the grid square are merely added to the coordinates 
of the corner of the square, thus giving the coordinates of the 
point from the origin. 



MAP READING 



75 



O 


X 


O 


CD 


CQ 


Ll_ 






c 


M 














' 


' 


UJ 


















n 


3 






















































c 


3 


















s 


1 






























□ 


T3 






















D 


J 






















■V 


r 








































f 






tf 


■^ 


r 


i 


u 


■) 


-- 




1 














c 


j 


tp 



tf 






For the purpose of measuring the coordinates of a point in 
the grid square, a scale may be made on two edges of a card, though 
a preferable form is one scratched on thin celluloid. 

The above description is far from complete, in that the rela- 



76 MAP READING AND TOPOGRAPHICAL SKETCHING 

tion of map reference to map projection has barely been men- 
tioned, whereas the two subjects are intimately related. 

There is one point in this connection, however, which must 
be mentioned. The grid lines printed on a map are parallel to 
each other, whereas on any map projection fit for military use 
the lines of latitude are curves, and the longitude lines or true 



H— h 



e 



c 



1 1 1 1 1 1 1 1 1 



b -- 



d 



I I I | i I I 



Fig. 28. — Location of Points in Grid Squares. 



meridians are converging. Hence it is that as we recede from 
the origin of any grid system, its lines depart more and more from 
the meridians of the map. This leads to the use of the term 
" grid north," which is the direction of the grid lines, differing 
from the magnetic north shown by the needle and the true north 
shown by the direction of the projection lines of the map. The 
angle between grid north and the meridians of the map, called 
the gisement of the grid, is one of frequent use in working with 
maps showing map reference grids. 



CHAPTER IX 

MAP READING— Continued 

Directions and Positions on Maps 

ORIENTATION 
Direction. 

The standard direction is the true north. The direction of 
any line is the horizontal angle which it makes with the standard 
direction. 

Determination of Direction. 

The Magnetic Needle. The only instrument readily available 
at all times for the determination of direction is the magnetic 
needle or compass. The needle is subject to certain minor varia- 
tions, and may be locally disturbed by the presence of magnetic 
bodies, but in any one locality and for any given year, the mag- 
netic needle may be said to point always in the same direction. 

The direction indicated by the needle differs in different 
localities, and in the same locality this direction is subject to 
slow but regular changes. The angle between the true north 
and the magnetic north is called the magnetic declination. 

In order to refer to the true north directions determined by 
the needle, it is necessary to know the magnetic declination for 
the locality and the time in question. This is usually known 
locally or is a matter of easily accessible record. If not, the 
true north must be determined by observation, from which the 
local magnetic declination can be determined. 

77 



78 MAP READING AND TOPOGRAPHICAL SKETCHING 

Accurate observations for true north must be made by the 
transit, by methods described in text-books on surveying and 
not necessary to repeat here. It is convenient, however, to be 
able to determine the true north approximately by Polaris, and 
roughly by the Sun. 

Approximate North by Polaris. Polaris is a star whose 
angular distance from the true pole is about 1° 18'. It can 
be readily identified by the " pointers " in the Big Dipper, and 
should be remembered in its relation to the pole, the Big Dipper, 
and the constellation Cassiopeia, as shown in Fig. 29. Note 
that the true pole is on the line with Polaris and the two stars 







/- True Pole 
■''V^Po/ar/s 
\°/8' 



Fig. 29. — -Approximate North by Polaris. 

indicated. When this line is vertical, Polaris is on the meridian 
or true north. When the line is horizontal, the azimuth or direc- 
tion of Polaris is approximately 1° 30' at latitude 40 degrees, 
increasing about 1 minute for each degree increase of latitude. 

The direction of Polaris can be established by suspending a 
plumb line from a height of about 8 feet, and arranging a sighting 
point a few feet in rear. If the observation is made when the 
line A B, Fig. 29, is vertical, the line established is the true north; 
if horizontal, the line, established will be 1° 30' from the true 
north on the side away from the Big Dipper. If the line estab- 
lished be prolonged and read by the compass, the magnetic declina- 
tion will become known approximately. 



MAP READING 79 

Approximate North by the Sun. With a watch lying flat, 
point the hour hand in the direction of the sun, using a match or 
pin held in a vertical position to cast a shadow if necessary to 
get the correct direction. Half-way between the hour hand and 
the XII mark will be the south point. This method is inapplicable 
in low latitudes where the sun is nearly overhead. A second 
method is to mark at about 9:00 a.m., the point where the top 
of a vertical pole casts a shadow. Describe a circle on the ground 
with the vertical pole as a center and the distance to the marked 
point as a radius. Watch for the shadow of the top of the pole to 
cross the circle in the afternoon, and mark the point where it 
crosses. Bisect the line joining these two points, and the point 
so determined will, with the base of the pole, mark a north and 
south line. This method really determines roughly when the 
sun is at equal altitudes morning and afternoon. Half-way 
between, it was on the meridian. 

Orientation. 

Orientation is the process of determining direction. Orienting 
a map means setting it in such a position that the north lines of the 
map point to the true north. Then all lines on the map are paral- 
lel to the corresponding lines on the ground. 

Orientation of a map may be accomplished by (a) setting the 
north line of the map in its correct direction by compass, or (6) 
by setting the map so that the line joining two points on the map 
coincides in direction with the line joining these same points on 
the ground. The subject of identifying on the map the position 
of points on the ground, and of identifying on the ground points 
shown on the map will be discussed later. For the present it 
will be assumed that such identification can be accomplished. 

Orienting a Map by Compass. Any map fit for military pur- 
poses will show on its face the meridian lines, and will usually 
show the map direction of magnetic north, sometimes only the 
local data for the magnetic declination. If there is no magnetic 



80 MAP READING AND TOPOGRAPHICAL SKETCHING 

north line on the map, one should be drawn before the map is 
taken into the field. In other words, if it is intended to use com- 
pass orientation, there is as little excuse for forgetting to prepare 
the map for the purpose as there is for forgetting the compass. 
A little forethought is better than a lot of ingenuity. 

Assuming, then, that there is a magnetic north line on the map, 
it is only necessary to set the N-S line of the compass parallel to it 
and shift the map until the needle is on the mark, when the map 
is correctly oriented. 

Orienting a Map by Identified Points. Assume that two 
points a and b on the map have been identified as points A and B 
on the ground. Set the map at A and turn it until the line ab 
points in the direction of B. The map will then be oriented, 
and all lines on the map will be parallel to the corresponding 
lines on the ground. If it be desired to identify any other point 
X on the ground represented by x on the map, sight along the line 
ax on the map and the point X will lie in that direction. 

If a map be set up over a known point and oriented by com- 
pass, the same results will be obtained, except that orientation by 
known points is more accurate. 

Location of Points on the Map. 

This subject has already been discussed to some extent in 
Chapter III, in connection with the subject of measurement of 
ranges for artillery and infantry. Some further phases of the 
subject will now be discussed. 

General Accuracy of Maps. 

In time of peace in the study of problems in strategy and 
tactics, the maps available will be general topographical maps 
resulting from formal surveys. At the opening of war, and at 
times thereafter when the scene of operations is shifting rapidly, 
the same character of maps will have to be used. In stationary 
or position warfare, there will be available both time and means 



MAP READING 81 

for the creation by the armies themselves of the special maps of 
large scale that are needed. The latter has been the case in many 
of the operations of the European War. 

In either case, military men will have to deal with maps the 
general character and accuracy of which may be .learned by 
inquiry concerning the date when the survey was made, the 
scale upon which the field work was executed, and the methods 
used in making the survey. 

If the maps have already been in use for purposes of military 
operations, their maximum error in the location of points and their 
general trustworthiness as to representation of horizontal and 
vertical detail will have become a matter of common knowledge. 
These facts concerning the general accuracy of the maps should 
be learned by inquiry rather than through the perhaps costly 
process of experience. 

Starting then with such information as can be gained by 
inquiry, the results of experience in the use of any given set of 
maps should be constantly kept in mind to check up or revise the 
estimate of their general accuracy. 

Representation in Plan and in Elevation. 

It is to be borne in mind that representation in plan, i.e., 
roads, streams, houses, etc., is practically entirely independent of 
representation in elevation by contours. Furthermore, repre- 
sentation in plan calls for no particular exercise of the imagina- 
tion on the part of the topographer, whereas representation in 
elevation calls for both imagination and estimation, and the results 
achieved will vary greatly with the skill of the topographer. 

A sharp distinction should therefore be drawn between the plan 
of the map and its contours in considering its accuracy. 

In a general way, the plan of a map resulting from survey 
methods may be accepted as accurate within the limits imposed 
by the scale of the field work, and with due caution as to the date 
when the survey was made because of possible later changes in 



82 MAP READING AND TOPOGRAPHICAL SKETCHING 

roads, houses, and vegetation in general. The addition of hori- 
zontal detail to a map from airplane photographs can be done with 
a good degree of accuracy, because of the checks from roads and 
other points known in plan. 

The representation of ground by contours is a very different 
problem. This representation is badly cramped on all except the 
large scale maps, and it is therefore generally impossible to draw 
the minor forms even if the necessary measurements could be 
made to locate and delineate them. Furthermore, there are 
ground forms over the whole area of the map, whereas the hori- 
zontal detail is limited. From these causes it results that the 
representation in elevation is far inferior to that in plan even in 
the larger scale maps, and this inferiority is accentuated as the 
scale becomes smaller. There is the additional difficulty that air- 
plane photographs are of little assistance in the representation 
of ground forms. Vertical photographs reveal practically nothing 
of the shape of the ground, and oblique stereoscopic views but 
little more. Thus the airplane photographs serve to correct the 
representation in plan within the enemy's lines, but not in relief. 

Identification of Points on the Ground. 

From the above discussion, it is apparent that in the process 
of identifying on the ground points shown on the map, the points 
selected should be those in plan rather than those delineated by 
contours. And of those shown in plan, the order of accuracy 
will be: 

1. Triangulation points; 

2. Points on principal traverses; 

3. Points on secondary traverses; 

4. Interior points. 

Finding Your Own Location on the Map. 

If it be required to find your own location accurately on the 
map, then the problem is that of plotting your position by suitable 
measurements to points that can be accurately identified. 



MAP READING 83 

The problem is frequently that of finding approximately 
your position on the map, in order that the map may be oriented 
and the ground studied from this point in connection with the map. 

The problem of finding your own location on the map varies 
greatly with the scale and detail of the map, as will be explained 
later. It may be assumed, however, that the problem is never 
wholly without known factors. Thus, if you are on the ground 
with the map in your hand, it is to be presumed that you remem- 
ber something of how you got there and where you came from, 
otherwise there may be some difficulty in finding your location 
on the map. 

In the problem of finding your location on the map, it may 
properly be assumed that you know approximately where you 
are to start with. Then it is only a question of identifying on 
the map the features which you see around you. If you have 
been following a road, this road is on the map and may be 
identified. The direction of the road, stream crossings, houses, 
and ground forms are examined on the map to see where you 
are. If the map is so poor that the features cannot be identified, 
or if you know so little of maps that you cannot recognize the 
map representation of these features, the situation is practically 
hopeless. Either a new map or a guide is needed. 

In case you desire your map location off the road, the point 
where you leave the road should be identified, the map oriented 
roughly by the direction of the road, and some decision made as to 
. pproximately where you intend to go, for instance to some hill 
or valley or any other selected point. The direction and dis- 
tance to the desired location are then known, and you will know 
by your watch about when you will get there. Knowing approx- 
imately where you are on the map, it is again merely a question 
of identifying the features about you. 

The safe course in finding your location on a map is to keep 
track of where you are. A little care will save a lot of hunting 
on the map. 



84 MAP READING AND TOPOGRAPHICAL SKETCHING 

Relation of Map Scale to Detail. 

There is perhaps nothing in the whole subject of map reading 
more important than a knowledge of the relation that exists 
between the scale of a map and the general character and accu- 
racy of the detail shown on it. 

Horizontal Detail. The conventional signs which represent 
the horizontal detail must be drawn large enough to be seen 
and recognized easily, for military maps must frequently be 
used where the light is poor. The amount of detail that can 
be shown in a given map area is thus strictly limited. 

Difficulty in horizontal representation begins to be encountered 
on maps of surprisingly large scale. It might be thought that 
on a map scale of 6 inches = 1 mile, or its metric equivalent 10 ,ooo > 
the 36 square inches representing each square mile would offer 
ample space in which to draw all the desirable detail. 

On the scale 6 inches =1 mile, -go of an inch represents a 
distance of approximately 18 feet. Thus a road or a house 
can actually be drawn to scale, though it is desirable to draw 
them approximately double their scale dimensions in order that 
they may be seen easily. A double track railroad symbol occu- 
pies a scale width of about 50 feet, so that it becomes evident 
that there is none too much room on this scale. 

But it is in the vegetation symbols that the difficulty becomes 
more evident. The symbols for orchards, corn, cultivation, 
grass, woods, and the like, can hardly be readably executed in 
a space less than about \ inch square, yet \ square inch repre- 
sents a ground area of about 5 acres on a scale of 6 inches = 1 
mile. 

From this it is seen that this scale permits of the accurate 
representation of the roads, railroads, houses (except in vil- 
lages), and stream courses, none of which, with the exception 
mentioned, occur in such numbers as to cause difficulty on 
account of the exaggerated scale of their conventional signs. 
But in the woods, orchards, cornfields, etc., it is seen how im- 



MAP READING 85 

possible it is to represent the ordinary farm orchard, the small 
clumps of trees, the two-acre cornfield, and the like. The 6 
inches = 1 mile scale must therefore be understood as showing 
only the wooded areas which exceed 5 acres in extent ,and even 
then their outlines are not necessarily accurately shown. The 
presence on the ground of many small clumps of trees and of 
much other horizontal detail of minor character not shown on 
the map should, therefore, occasion no surprise. 

As the scale of the map decreases, the difficulties of repre- 
sentation increase rapidly. More and more of the less important 
detail must be left out, and that which remains must be more 
and more generalized. 

If we take 1 inch = 1 mile as a typical small scale map, the 
difficulties of representing horizontal detail become apparent. 
The railroads will usually be not so numerous as to cause dif- 
ficulty, and their alignment can generally be closely followed, 
but yards, switches, cuts, and fills can no longer be shown. 
Isolated houses are usually shown, but the straggling village 
has to be represented by a few house symbols. The principal 
roads can be shown, but many of the minor changes in direction 
and sinuosities must be omitted. Many of the minor roads 
may be crowded out, and usually no attempt is made to show 
trails and paths, except in mountainous country where communi- 
cations are few. Vegetation symbols are usually omitted entirely, 
and if used show nothing more than the location and general 
outline of the principal forest areas. Thus in the 1 inch = 1 
mile map, the plan is stripped almost bare of detail. 

One important point must be remembered. The adequacy of 
horizontal representation on any scale is dependent upon the 
character of the country represented. If there is little detail 
to be shown, if there is but an occasional railroad, if roads are 
few, and if other detail is similarly lacking, a small scale may 
be adequate for representation, whereas a thickly populated region 
with its multiplicity of communications and other detail may 



86 MAP READING AND TOPOGRAPHICAL SKETCHING 

require considerable generalizations and omissions even on the 
larger scales. The general character of the country must there- 
fore be taken into account in judging how faithfully it can be 
represented on any given scale. 

Vertical Detail. The representation of the ground forms by 
contours has been seen to involve imagination and estimation 
on the part of the topographer. In small surveys, the contours 
may be run out and located instrumentally, but in general topo- 
graphical surveys the topographer must locate a few points in 
position and elevation, plot these on the map, and then draw 
the contours. In the process the topographer is hampered by 
the limitations of the scale upon which he is working, and this 
limitation is just as severely felt in contour representation as 
it is in the representation of horizontal detail. 

If we consider a map scale of 6 inches = 1 mile, there is the 
same difficulty that 5V of an inch represents 18 feet, and this 
is accentuated by the fact that the contours must be drawn 
free-hand. It is therefore apparent that there is a great deal 
of minor vertical detail that cannot be shown by contours on a 
scale of 6 inches = 1 mile, or if attempted to be shown the result 
would be attributed to a tremulous hand. 

In the representation of the ground by contours, the topographer 
is under the painful necessity of making some statement with 
respect to the form of the ground at every point on the map. He 
cannot discontinue the contours, and wherever they go they say 
what the shape of the ground is, and what is the degree and direc- 
tion of the slope. 

As the scale of the map decreases, the difficulties of contour 
representation increase. Soon a point is reached where all of the 
smaller valleys and spurs must be ignored, and only the main hill 
and valley features portrayed. 

A very vivid illustration of the effect of the map scale on both 
horizontal and vertical detail may be had by comparing a large 
scale map with a small scale map of the identical area. Thus in 



MAP READING 



% 



87 



Fiji;. 30, the area represented in the left-hand part of the figure 
on a scale of 6 inches to the mile is the same as that shown in the 
dotted rectangle in the center of the 1-inch to the mile map 
shown in the right-hand part of the figure. The V.I. of the 6 inches 
= 1 mile map is 10 feet, that of the 1 inch = 1 mile is 50 feet. These 
maps serve to shown that practically all of the detail has dis- 
appeared in the 1 inch = 1 mile scale, and it is seen how impossible 
it would be to locate any position on the 1 inch = 1 mile map by 
identifying it by the shapes of the contours except in the case 
of sharp summits. 




Fig. 30. — Relation of Scale to Detail. 



The difficulties of representation on the map, both of hori- 
zontal and vertical detail, can be learned best by attempting the 
work in practice. After such practice, the student will be better 
prepared to make proper allowance for the inaccuracies and 
generalizations that exist in the best of topographical maps, and 
will understand better the limitations in the uses of maps. 

Use of Compass with Map. 

The orientation of a map by compass has been described. 
The other principal use of the compass in connection with maps 
is for the purpose of following on the ground either by day or by 



88 MAP READING AND TOPOGRAPHICAL SKETCHING 

night, a line laid out on the map. Usually this line will be the 
course to be followed on the ground to reach a given point. It 
may be either straight or broken, but there should be as few 
changes of direction as possible, particularly if the course is to be 
followed by compass at night. 

When such a course is laid out on the map, the measurements 
of the lengths and directions of its different courses must of 
necessity be referred to the lines printed on the map — either the 
true north lines or the lines of the grid. It is necessary then to 
be able to translate such a description into terms of compass 
readings and distances. 

Compass Bearings. The compass bearings or magnetic azimuths 
will differ from the true azimuths by the amount of the magnetic 
declination. It has been seen that the data with respect to the 
magnetic declination may be on the map, may be learned by 
local inquiry, or may be determined on the ground, and they 
are therefore assumed to be known. 

The azimuth or true bearing of a line is the horizontal angle 
between it and the true north line. These azimuths are measured 
continuously around the circle from the south point around 
through west, north, and east. As indicated in (a), Fig. 31, the 
azimuth of directions between south and west are from to 90 
degrees, between west and north, 90 to 180 degrees; between 
north and east, 180 to 270 degrees; and between east and south, 
270 to 360 degrees. The north point may be used as the origin, 
measurement being made in the same direction. Which origin 
is used will depend upon the graduation of the compass to be used. 
Thus if the compass is graduated from the north as zero, the 
azimuths of the courses to be followed should be similarly 
calculated. 

The relation of the magnetic azimuth to the true azimuth will 
be seen by reference to (6), Fig. 31. Let us assume that the 
magnetic declination is 14 degrees west, its present value in 
northern France. Then if the true azimuths are anywhere between 



MAP READING 



89 



and 346 degrees, the magnetic azimuth will in each case be 14 
degrees greater. Particular note must be made of the true 
azimuths between 346 and 360 degrees. It will be seen that the 
line with true azimuth 347 degrees has a magnetic azimuth of 

1 degree, 348 of 2 degrees, etc., and for the true azimuth degree, 
the magnetic azimuth is 14 degrees. 

Some compasses are graduated in quadrants, which compli- 
cates the situation considerably. A full discussion is unnecessary. 




E W 

M.W. 




Fig. 31. — True Azimuths. 



A diagram like (6) Fig. 31, will show how to convert true azimuths 
into magnetic azimuths, or bearings. 

Instead of the angle with the true north being used, it may be 
more convenient to use the angle with the north lines of the map 
grid, called grid north. In this case the angle between magnetic 
north and grid north is known or measured, and the magnetic 
bearing's are calculated as before. 

Marching by Compass. • Having ascertained the compass 
bearings and lengths of the courses to be followed, the problem 
is to follow these courses either by day or night as may be required. 



90 MAP READING AND TOPOGRAPHICAL SKETCHING 

Following the course by day offers little difficulty. With the 
compass in position for reading its bearing, turn until the desired 
bearing is indicated, and the sighting line will show the direction. 
Note some distant point on this line upon which to direct the 
march. Follow this direction until the turning point is reached. 
This may be recognized by its map delineation, but the distance 
traveled should be kept track of as a check. Lay out the next leg 
of the course in the same way. 

Marching by compass by night is more difficult. A compass 
with luminous markings must be used to find the direction. If 
there is enough light to be able to pick out some point on the 
horizon to march by, this is done. Otherwise one man with a 
spot of luminous paint on his back is sent forward a short distance 
and placed on the line by compass. Then the man with the 
compass goes to the spot thus marked and prolongs the line 
in the same way. Careful track is kept of the distance cov- 
ered, as turning points cannot be recognized in the darkness and 
can only be known by distance. 

If this process of following a compass course at night is to 
be used to guide marching troops, men must be posted at inter- 
vals along the route to keep touch between the compass man and 
the head of the column of troops. These men open out or close 
up as may be necessary to preserve the connection between 
the compass man, who moves forward irregularly, and the troops 
following at a regular gait. 



CHAPTER X 
TOPOGRAPHICAL MAPPING 

Instruments and Methods 

Mapping Methods. 

The methods by which horizontal detail is located may be 
dismissed with little discussion. It requires no imagination to 
see a road, a stream, or a house. To locate these, all that is 
required it to make the necessary measurements with whatever 
instruments have been provided, plot the point or points so 
located, and draw the conventional signs representing the proper 
objects. The accuracy of the measurements will depend upon the 
instruments used, but a very fair degree of accuracy can be secured 
by skill and care, even with improvised instruments. The accu- 
racy with which the plotting can be done and the amount of 
detail that can be shown are both dependent upon the scale of 
the map. 

The representation of the form of the ground is a very dif- 
ferent and much more difficult matter. It is impossible to take 
the time to mark out contours by points located with a level, 
and then to make the measurements necessary to plot the contours 
so marked. 

All that is possible in the general case is to locate in position 
and elevation a comparatively few points, and from these few 
known points the topographer must draw the contours to repre- 
sent the ground forms. The representation will be easier and 
more accurate if the points which are located are chosen so as 
to give the best opportunity to see the ground forms. 

91 



92 MAP READING AND TOPOGRAPHICAL SKETCHING 

Even in elaborate surveys, where the primary control is 
by triangulation, the secondary control by instrumental traverses, 
and the filling-in is done by instrumental methods, the representa- 
tion of the ground forms by contours depends finally upon the 
ability of the topographer to draw the contours with only a 
few located points to guide him in this work. 

There are thus two fundamental elements in topographical 
work : 

(a) The location of points in plan and elevation. 

(b) Representation of ground forms from located points. 

The location of points in plan and elevation is merely a matter 
of instrumental measurement, requiring only facility in the 
use of instruments and ability to make calculations and plot 
the results. The instruments by means of which points are 
located in plan and elevation in sketching methods will now 
be described, together with the actual methods of plotting lines 
and making measurements with these instruments. 

Standard Sketching Equipment. 

Military mapping must frequently be done in a minimum 
of time. The equipment used and the methods adopted must 
therefore be such as to afford results of reasonable accuracy 
with economy of time. As in all other military operations, 
economy of labor is of secondary importance. 

As a result of all the attention given to the subject of topo- 
graphical methods, opinion in our military service finally crys- 
tallized in favor of the plane-table method of sketching. The 
advantages of the plane-table as an instrument for topographical 
filling-in are well known. It affords the means of making all 
necessary measurements, furnishes the plotted positions of the 
selected (control) points immediately, and enables the topo- 
graphical representation to be done by the topographer on the 
ground. 

The standard plane-table is, however, both heavy and bulky. 



TOPOGRAM K AL MA PPING 



93 



The military equipment for topographical sketching must be 
light and easily portable, even at some sacrifice of accuracy, and 
must be available for mounted as well as dismounted use. 




Fig. 32. — Standard Sketching Board. 

Balancing these considerations, the equipment adopted for 
topographical sketching is the Engineer Sketching Board and 
its accessories, as shown in Fig. 32. The full materiel composing 



94 MAP READING AND TOPOGRAPHICAL SKETCHING 

a sketching unit is packed for field service in a convenient carrying 
case, which carries also a full reserve of paper and other supplies. 

The principal items of this equipment are the sketching board 
and tripod, the alidade, and the clinometer. 

Sketching Board and Tripod. The sketching board is of 
white pine, 14 inches square, with reinforce strips on the ends 
to prevent warping and splitting. A trough compass with 3|- 
inch needle is let in flush with one edge. There is a small lever 
for raising the needle from the pivot. On the corners of the 
board are 4 clamp screws for securing the paper to the board. 
Along the edge by the needle are stamped an inch scale and 
plotting slope scales. On the opposite edge is a tangent scale 
with center near the edge of the compass box, from which a 
weight may be suspended and the tangent scale used for reading 
slopes. The edge of the board is the sighting edge for this pur- 
pose. At the corners of the board are holes for a carrying cord. 
In use, the carrying cord should pass from the outer left-hand 
corner of the board over the left shoulder and under the right 
arm to the inner right-hand corner of the board. The cord 
is adjusted to hold the board at a convenient height for use. 

The tripod is of the folding type, substantially made of hard 
wood. The two pieces forming the upper joint of the leg swing 
outward on hinges and are sprung inward and engaged on two 
pins on a lug on the tripod head. When engaged these pieces 
are held in position by a spring clip. The lower joint of the 
leg slides outward and is clamped by a screw. 

The tripod head is of three-ply veneer, has three lugs for the 
attachment of the legs of the tripod, and is provided with a 
clamping screw to attach the board to the tripod. The upper 
surface of the tripod head is provided with a felt pad for security 
in clamping. 

Alidade. The alidade is of boxwood, 9| inches long, suitably 
weighted with lead, and of triangular cross-section. The ex\gv* 
are faced with celluloid, and upon these faces are stamped an 



TOPOGRAPHICAL MAPPING 95 

inch scale, scales of hundreds of yards for 1, 3, and- 6 inches = 
1 mile, and plotting slope scales. Two edges are left blank for 
the addition of a stride scale and trot and walk scales. 

Clinometer. The service clinometer is non-magnetic and of the 
pendulum tj^pe, provided with stop and release. The scale is 
attached to the pendulum and the part of the graduation near 
the line of sight is reflected in a small mirror so that it can be 
seen while sighting with the clinometer. The line of sight is 
marked by a black line showing on a piece of white celluloid at 
the edge of the sighting opening. The scale is graduated in 
degrees, the graduation being in red on the plus or upward slopes, 
and in black on the negative or downward slopes. Slopes can be 
read by estimation to the nearest quarter of a degree. The 
clinometer is tested for mechanical condition by watching the 
swing of the arc when released to see if it swings freely and runs 
true. The clinometer should be tested daily for index error by 
reading forward and backward as accurately as possible on a 
selected slope. The mean of the readings is the correct degree 
of slope, and from this the index correction is determined, + if 
the upward reading was smaller than the mean value, and —if 
larger. The index correction is added algebraically to all 
readings. 

Accessories. A pace tally forms part of the equipment, but 
its use is not recommended because it is not made of non- 
magnetic metal. A woven web pocket is provided, with suitable 
places for carrying the alidade, eraser, and pencils. The eraser 
is more conveniently carried attached to a clamping screw at one 
corner of the sketching board by a string about 15 inches long. 
There is a timing pad for use in mounted work. A pad of sketch- 
ing paper and an envelope containing sheets of celluloid with 
ground-glass surface on one side complete the equipment. 

Improvised Sketching Equipment. 

If necessary to use improvised equipment, it should approxi- 
mate as nearly as possible to the standard equipment. 



96 MAP READING AND TOPOGRAPHICAL SKETCHING 

The board can be provided without trouble. A pocket com- 
pass can be permanently secured to the board in any convenient 
manner, usually in a recess cut out for the purpose. The method 
of attaching the compass will depend upon the type of compass 
available. The needle stop must be left accessible. 

The triangular alidade can be improvised and may well be 
only 6 inches long. The main difficulty will be its lack of weight 
and the consequent likelihood of being blown off the board. The 
necessary scales are made and pasted on its edges. The slope 
board can be used as a substitute for the clinometer, and with 
careful use will give quite as accurate results. The main difficulty 
in the use of the slope board is that vertical angles cannot be 
read while the board is in position, and consequently all vertical 
angles must be read either before the board is set up or when 
ready to leave the station. This is merely a matter of incon- 
venience, however, but involves loss of time if any horizontal 
angles happen to have been omitted, as the board must then be 
set up again and oriented. 

The tripod offers the greatest difficulty in the improvisation. 
Rotation and clamping should be provided for if possible. A 
camera tripod can be used, but generally its metal fittings are of 
magnetic metal. If a tripod or jacob-staff be improvised, brass 
screws should be used and not nails. Or the tripod idea may be 
abandoned and the carrying cord used. In this case, for important 
shots the board is removed from the carrying position and placed 
on the ground for orientation and sighting. 

In general, the crudeness of the improvisation must be com- 
pensated for by additional care in the use of the instruments. 
The methods described for using the standard equipment apply 
as well to improvised equipment. 

Using the Standard Sketching Equipment. 

Sketching Paper. If tracing vellum or celluloid is used, it is 
necessary to put a piece of white paper (preferably bristol board) 



TOPOGRAPHICAL MAPPING 97 

underneath, otherwise the pencil lines cannot be seen against 
the board. For instruction purposes, a good grade of plain 
white paper can be used as well. Some practice should be had 
on the vellum and celluloid to learn the little peculiarities of both. 

For the vellum, an F or HB pencil should be used. All 
temporary lines, such as plotted directions, elevations of points, 
and tentative contours, should be drawn in light lines so that 
they may be erased easily and cleanly. Permanent lines of the 
sketch, such as roads, fences, final contours, and the like, should 
be drawn with enough pressure (determined by test) to prevent 
clean erasure. On sketches so drawn the temporary lines can be 
erased without fully erasing the permanent work, which can then 
be restored. 

For the celluloid, a 6H pencil is to be preferred, kept sharpened 
to a fine conical point. Frequent sharpening is necessary because 
the ground-glass finish of the celluloid wears away the point 
rapidly. A small sand-paper block should always be attached 
to the leg of the tripod by rubber bands for use in pointing the 
pencil. The tentative lines are drawn in light lines, permanent 
lines somewhat heavier, but both will disappear under the eraser. 
Erasing of tentative lines must therefore be done in small patches, 
the permanent lines being restored. Weight of line is secured on 
celluloid by going over the lines two or three times. For the 
finish of strong black lines, a softer pencil (2H) should be used. 
Avoid too heavy a pressure on the pencil, as it will cut a furrow 
in the celluloid, and several such lines close together will distort 
and buckle it. 

Assembling the Sketching Board. The sketching board is 
dressed for use by attaching the paper, assembling and attaching 
the tripod, fastening a good eraser to one of the clamping screws 
by a string, and fastening a sand-paper pad or block to one of the 
legs of the tripod. Necessary pencils, alidade, extra eraser, and 
knife are carried in the web pocket, if available, otherwise in the 
pockets. The clinometer is carried in its leather case upon the 



98 MAP READING AND TOPOGRAPHICAL SKETCHING 

belt or in the pocket — never in the pocket without the case, or 
lint will get inside and put it out of commission. 

Carrying the Sketching Board. Thus equipped, the sketching 
board is carried to the point where work is to be started, either 
on the shoulder or with the tripod under the arm. The latter 
is preferable in brush or woods. If carried on the shoulder, care 
must be taken that the board does not become loosened and fall 
off. If you hear anything rattling, see if the needle is properly 
clamped. It is ruinous to carry the needle loose on the pivot. 

Setting Up the Sketching Board. At the starting point, the 
board is to be set up and oriented for use. Proper methods 
contribute greatly to the speed of work, and detract in no wise 
from accuracy, hence the methods described should be followed 
exactly. 

In setting up the board, take one tripod leg in each hand and 
swing the free leg forward and place it on the ground. Now 
place one of the remaining legs of the tripod on the ground about 
2 feet from the first. With the remaining leg level the board. 
Lateral movement will tilt the board sidewise, radial movement 
in the opposite direction. On sloping ground, always set two 
legs on the same level, the odd leg being extended uphill. 

Orienting the Sketching Board. Before setting up the board, 
always look around to see if there is any object near which might 
disturb the needle. If so, the position must be changed. If 
the north is approximately known, turn the board so that the 
north end of the needle points approximately north, and then 
release the needle. If the north is not known, find the setting 
of the board by trial. The clamping screw of the tripod head 
should be set so that the board can be twisted by some little 
force, too much to permit the board to be displaced in use. 

After the needle is released, the board is set approximately, 
so that the needle swings about equally each side of the black 
orientation line in the bottom of the compass box. After the 
needle has settled completely, the final adjustment of the orien- 



TOPOGRAPHICAL MAPPING 99 

tation is made. In doing this, the eye should be placed directly 
over the needle, and care taken to see that the extreme N. end 
of the needle is exactly in the middle of the orientation line. 
In this process of orientation, watch the needle carefully to see that 
it is not in contact with the glass, thus restricting free move- 
ment. It is an excellent plan to tap the corner of the sketching 
board either before or after the orientation is complete. If the 
needle is free and the pivot in good condition, the needle shows a 
characteristic quiver when the board is tapped. If the end of 
the needle is in contact with the glass, tapping the board causes 
the end in contact with the glass to fly down. 

Alidade. The triangular alidade has two distinct uses: 

1. As a sighting line and ruler; 

2. As a plotting scale for both distances and slopes. 

First Use. Assuming that the board has been set up and 
oriented, and that a point has been marked on the paper with 
a small circle drawn around it to indicate that it is a traverse 
station, everything is in readiness to plot the directions of any 
lines that may be desired in connection with the sketch. 

To determine and plot the directions, it is necessary to place 
the alidade in position on the board, with its edge accurately 
upon the traverse station, and then, keeping the edge of the 
alidade over the station, to point the alidade so that by sighting 
along its upper edge the distant object is seen accurately in 
prolongation of it. 

The difficulty is in keeping the edge of the alidade over the 
station while making the pointing. The pointing should be made 
with the eye at the level of the board, whereas the eye must 
be above this level to see if the edge passes through the point. 
The best method is first to point the alidade as nearly in the 
direction of the object as can be done while looking down on 
the board. As an aid, point the finger at the distant object 
and adjust the alidade parallel to it. Then take a squatting 



100 MAP READING AND TOPOGRAPHICAL SKETCHING 

position with the eye near the edge of the board and correct 
the pointing; rise slightly so that the point can be seen and 
adjust to the point without disturbing the direction; check the 
pointing and repeat if necessary. Draw a line along the edge 
of the alidade through the traverse station, and this is the plotted 
direction of the distant point. If the sight is much inclined, 
set a pencil on the board in a vertical position at the forward 
end of the alidade if sighting upward, at the rear end if down- 
ward, to assist in making the pointing. Watch the pencil to see 
that it remains vertical. 

Second Use. The alidade is used to plot the distances that 
are paced during the work of sketching. The length of stride is 
computed and the working scale of strides is constructed as 
described in Chapter IV. This scale is now transferred to the 
edge of the alidade. After distances have been paced, the edge 
of the alidade is placed along the plotted line and the number 
of strides is plotted from this stride scale. 

It has been seen that the edges of a standard alidade are 
marked with plotting slope scales. These slope scales show, 
on a scale of 6 inches = 1 mile, the map distances between 10-foot 
contours for various degrees of slope. These scales may be used 
either for marking directly the distance between contours on 
slopes, or for determining the elevation of any desired point which 
has been plotted in horizontal position on the sketch. 

In plotting the actual contour points with the slope scale, 
the plotting must start from some point on the slope the ele- 
vation of which has been determined, as say, 226 feet. Suppose 
the slope upward from this point is 3 degrees. Then the 3-degree 
scale is applied in the direction of the slope with -jo °f the con- 
tour interval only above the 226-foot elevation, so that the 
first point marked will have an elevation of 230 feet. From 
there on the full 3-degree interval is plotted for the spacing. 

In plotting the elevation of points on traverses, or points 
located by intersection or other methods, the slope of the line 



TOPOGRAPHICAL MAPPING 101 

to the point must be read from some known elevation. Suppose 
in a traverse, that we have a station of elevation 387 feet. A 
sight is made to the next station forward, a slope of 2 degrees 
read, the distance paced, and the new station plotted in hori- 
zontal position. The 2-degree slope scale can be applied to this 
line exactly as if it were a uniform slope and the number of 10- 
foot intervals counted, thus determining the difference in ele- 
vation between the two points, the forward point being higher 
if the vertical angle is plus, and lower if minus. Instead of 
computing the difference of elevation between these points and 
then having to add or subtract this distance, it is more con- 
venient to set a partial interval, T 3 o in the above case, and then 
count forward by contour intervals and estimate the fractional 
interval at the end, so that the elevation of the forward point 
thus becomes known directly without any addition or subtraction 
other than has been done in counting up or down the slope. 

If a point has been located off the traverse by intersection 
from two points on the traverse and the slope read to the inter- 
sected point from each of the traverse points, the elevation of 
the intersected point could be determined by using the slope 
scale on the line from either traverse station. In practice it is 
computed from each of the stations and the mean value of the 
elevation used. 

Clinometer. In using the service clinometer, a very con- 
siderable degree of care is necessary to secure accurate results. 
The clinometer must be held in a steady position which is diffi- 
cult, particularly in a heavy wind. It is best to rest the arms 
against the body and the edge of the clinometer against the face, 
so as to get as much support as possible for the arms and hands. 
The pendulum is released by pushing back the locking slide and" 
then pushing down upon the release stop. The pendulum is 
free to swing only while the stop is pushed down. The stop should 
therefore be held down until the pendulum has come to rest and 
the reading has been completed. There is a certain amount of 



102 MAP READING AND TOPOGRAPHICAL SKETCHING 

jump of the scale noticeable when the stop is released, and this 
would affect the reading. In reading any slope with the clin- 
ometer it must be remembered that the clinometer is held at a 
distance of about 5| feet from the ground. To measure the actual 
slope of the surface of the ground, the sight must be made at a 
point as high as the eye above the ground. With such a reading 
the correct relative difference in elevation will be determined. 
If we stick rigidly to sighting at a forward point b\ feet above the 
ground, the angle so read will often include a quarter-degree 
reading or some other degree not found on the slope scale, 
as 6^, 2|-degrees, etc. The differences in elevation corresponding 
to a 6| -degree slope may be considered as composed of a 3| -degree 
slope added to a 3-degree slope. The difference due to each 
may be found separately and the two added. The 2|-degree 
slope may be considered as a 2-degree slope plus \ degree. The 
j-degree difference in elevation is computed by using four times 
the 1 degree interval on the scale. It is, however, a great sav- 
ing of time when the true reading is some degree and fraction 
not on the slope scale, to take advantage of this latitude offered 
by the height of the eye, to read an angle which is on the slope 
scale, correcting the result by what is seen through the clinometer. 
Thus, if the forward slope is plus 2\ degrees when the point 
sighted at is estimated to be at the height of the eye above the 
ground, it is well to see where the line of sight passes for a 2-degree 
slope. For a 2-degree reading, let us suppose that the line of sight 
passes at an estimated distance of 2 feet above the ground at the 
point sighted at. It is then merely necessary to remember that 
the point is actually 3| feet higher than it would have been if 
the slope had been exactly 2 degrees. It is much simpler there- 
fore to use a 2-degree reading and add 3| feet to the elevation. 

It is difficult to read very small slopes near degree. The 
difference in level in this case, except for very long shots, is only 
a few feet. This difference of elevation can probably be more 
accurately estimated if a 0-degree reading ( = level) is made 



TOPOGRAPHICAL MAPPING 103 

from the higher of the two points, and the distance above ground 
at which the line of sight passes at the lower point is estimated. 
The height of the eye must be subtracted from this estimated 
height to get the difference of level. If the difference of level of 
the two points is less than 5| feet, it is better to make the 0-degree 
reading from the lower point. 

A careful topographer will test his clinometer daily, handle it 
carefully, and never release the stop in such a way as to allow 
the pendulum to swing violently against its stopping lugs. Used 
carefully, reading the vertical angle forward and backward on the 
principal long shots, and taking the mean with careful pacing of 
distances, there should be no difficulty in keeping the error in 
elevation within 5 feet to the mile of traverse. Such an error 
will offer no difficulty in plotting the contours. 

Slope Board. In using the sketching board as a slope board 
for the measure of vertical angles, either the alidade or a lead 
bob is suspended by a string about 15 inches long passed through 
a hole near the compass box and knotted on the under side of the 
board. The lead bob is much to be preferred, because it is next 
to impossible to check the swinging of the suspended alidade if 
the wind is blowing. 

To read a vertical angle with the slope board (right-handed 
man), gather the legs of the tripod together and hold them in the 
left hand at the balancing point. Rest the left elbow firmly 
against the body, the feet spread well apart. Sight forward along 
the compass box edge of the sketching board to a point the height 
of the eye above the ground at the forward station. The right 
hand must be relied upon to find by the sense of touch when the 
bob has stopped swinging, to shield the bob from the wind, to 
see that the string is not rubbing against the edge of the board and 
thus damping the motion of the bob; and, when all of these 
conditions are satisfied, then to press the string against the board 
without slipping until the reading of the angle is made. This is 
a large contract for the right hand to fulfill. To accomplish it 



104 MAP READING AND TOPOGRAPHICAL SKETCHING 

the forefinger is placed against the lower edge of the board beyond 
the point where the lead bob will hang. The fingers are spread 
around the bob to protect it from the wind and the little finger 
used very carefully to touch the bob to see whether it is swinging. 
The thumb is used to touch the string lightly to be sure that 
it is swinging clear of the face of the board. When these con- 
ditions are fulfilled and the line of sight is correctly adjusted, the 
string is pressed against the board by the thumb, the tripod legs 
turned downward, so that the board is brought to a horizontal 
position, and the angle marked by the string is then read. This 
vertical angle is, of course, used in the same way as a clinometer 
reading. 

Skill in the use of the slope board requires considerable prac- 
tice. This practice should be continued until the same reading 
can be repeated without difference. 



CHAPTER XI 
TOPOGRAPHICAL MAPPING— Continued 

Traversing — Selection of Control Points — Contour 
Problems 

Traversing with the Sketching Board. 

By continuing the operation described in the preceding chap- 
ter, a traverse results. Starting from an initial point the eleva- 
tion of which is known or assumed, the sketching board is set up, 
levelled, and oriented. The directions to the point selected as the 
next traverse station and to such side points as it may be desired 
to locate are plotted, and the slopes are read along the various 
lines. If there are any slope lines that will be of value in plotting 
the contours, such lines are located by approximate pointings of 
the alidade. Either a memorandum record of slopes and identi- 
fication of points is made along the plotted lines, or the shots are 
lettered and the memorandum is made on the edge of the paper. 
Having completed this work, the needle is raised from its pivot 
(important), and the distance is paced to the next traverse station. 
Upon arriving at the station, the number of strides may be jotted 
down on the edge of the paper or remembered, and the board then 
set up, levelled and approximately oriented. The stride scale of 
the alidade is now used to plot the distance, and the second traverse 
station is marked and its elevation computed by using the slope 
scale and the vertical angle previously read. The orientation of 
the sketching case is now made exact. Intersecting shots are made 
on the points located from the first traverse station, and on such 
additional points as will be visible from the third station. The 

105 



106 MAP READING AND TOPOGRAPHICAL SKETCHING 

slope of each shot is read and recorded. The elevations of the 
points located by intersection from the first and second stations 
are computed. The needle is now raised from the pivot (still 
important), and the traverse continued at will. As a test of the 
accuracy of the elevations as well as of the traverse in plan, 
the traverse should be made finally to close upon the initial point. 
The error of closure, using the standard sketching equipment, 
should not exceed about ten strides per mile of traverse, and the 
error in elevation should not exceed about 5 feet per mile. With 
improvised equipment the errors may be greater due to diffi- 
culties of manipulation. The result of such a traverse will be the 
location of a series of points the elevation of each of which is 
determined, together with important slope lines. 

If the traverse should pass along the crest of some command- 
ing ridge, the plotting of direction may be made with special 
care, the pacing of the distance repeated to get the mean value, 
and a specia! effort made in every way to make this section of the 
traverse as accurate as possible. The two ends may then be used 
as a base line from which as many points as may be desirable 
are located by intersection, thus plotting the resulting points 
with approximately the same accuracy as the base line. Such a 
system of points will form the frame-work for a " base-line " 
sketch of the area. 

Locations by Resection. If the two ends of the base line 
are marked by some objects such as trees or telegraph poles 'that 
may be seen from points off to the side, then additional points 
can be located by the method known as resection. To locate a 
point by resection, the sketching board is set up and oriented 
accurately. Then a sight is made to one end of the base line 
with the edge of the alidade passing through its plotted position. 
A line is then drawn in the direction of the point occupied. 
Another similar shot is made upon the other end of the base line, 
and the intersection of the two lines so drawn marks the plotted 
position of the occupied point. The vertical angle to each end 



TOPOGRAPHICAL MAPPING 107 

of the base line is then read and the elevation determined by the 
use of the slope scale. 

Whether the whole system of points thus located by a traverse 
will be useful in making a contoured map of the area will depend 
upon the points that have been located. It is, therefore, very 
important to learn to select those points which will be of the 
maximum use in delineating the area by contours. Such points 
are called control points. 

Contour Problems. 

There is one class of problems which can be given indoors 
that forms an excellent preparation for the selection of control 
points in the field work by showing the sort of framework neces- 
sary for the delineation of the ground. These problems also give 
excellent practice in the working out of typical contour forms. 
Such problems are called contour problems, and the form of 
the data and the different steps in the solution of these problems 
are indicated in Figs. 33 and 34. 

It is to be noted that the data are given in the form of a plan 
of the drainage system of the area, together with the elevation 
of certain points along the stream courses and the location and 
elevation of certain points on the intervening high ground. The 
theory upon which the problems are solved is that the data give 
the location and elevation of all points where the contour spacing 
changes. 

If the slope along the stream course changes, the elevation 
at the point where such change occurs must be given. Similarly, 
if a change of slope occurs on the hills, the point where such change 
occurs must also be given. The location of the stream courses 
is the first and most important thing in the contoured delineation 
of an area, because upon the plan of the drainage system depends 
the whole shape of the contours along the stream courses, and 
these form the basis for working out the forms at the tops of the 
hills. 



108 MAP READING AND TOPOGRAPHICAL SKETCHING 



The method of solving these problems is indicated in the four 
stages shown in Figs. 33 and 34. 



i 


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650 


790 / 




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Second Stage 
Fig. 33. — Solution of Contour Problems. 

The first step is to determine, by interpolation from the eleva- 
tions given along the stream courses, the points where the contours 



TOPOGRAPHICAL MAPPING 



109 



cross the streams. This interpolation is done first along the 
principal stream courses, and then, using the elevations at the 




Fourth Stage 
Fig. 34. — Solution of Contour Problems — Continued. 

mouths of tributaries determined by interpolating between the 
contour crossings on the main stream, the elevations are similarly 



110 MAP READING AND TOPOGRAPHICAL SKETCHING 

interpolated along the tributaries. In the absence of any other 
evidence it may be assumed that the dry valleys indicated by 
broken line tributaries all head at about the same general elevation 
below the crests. Consequently, the distance between the con- 
tours along streams can be estimated by the length of the indicated 
stream course. Having thus indicated the points where the con- 
tours cross all the streams and indicated wet-weather stream 
courses, the interpolation is next carried out between the stream 
courses and the points on the ridges. This interpolation can 
be carried out both upon lines from the ridge points perpendicular 
to the streams and also along the ridge or spur lines which extend 
in the direction of the junction of the including streams. If there 
be no intervening stream courses, interpolation can also be car- 
ried out along the ridge lines, curved or straight, which connect 
the points of given elevation on the high ground. 

The result of all this interpolation is the location of such a 
number of points on each contour that the shapes of the contours 
themselves are very closely indicated, and if the type forms of 
valley, spur, and hill contours are followed, there will be little 
variation possible from the correct solution of the problem. 

The result of the solution of a series of problems of this kind 
will be to impress upon the mind of the student the fact that if the 
stream courses be correctly located and the points where the slopes 
of the streams change are indicated, then the locations and eleva- 
tions of a few points properly chosen on the intervening high 
ground will be sufficient for the contoured delineation of the area. 

If the ground to be represented is the result of erosion from 
sedimentary deposits, the existence of uniform slopes and convex 
crests is to be expected and this, as a matter of fact, is the char- 
acter of the country represented in Figs. 33 and 34, and also 
in Figs. 35, 36, 37 and 38, following at the end of this chapter, 
which are additional contour problems for solution by the student. 

If the area to be represented is of other geologic forms, as for 
instance igneous rock, then the uniform slopes and smoothly 



TOPOGRAPHICAL MAPPING 111 

rounded forms so convenient for the topographer are much less 
likely to exist. The ground forms will be intricate and irregular, 
and a great many more interior points will be necessary for a 
satisfactory delineation of the ground. Even in this character of 
country, however, it is only necessary to locate the stream lines 
and a much greater number of intermediate points to enable 
the ground forms to be delineated. 

In very intricate country, it may be necessary to resort finally 
to the ability to represent by contours a small area from any 
given point, and then locate enough points so that the represen- 
tation will be continuous. 

Contour problems for practice follow. The data for them 
should be traced on a piece of thin (or tracing) paper to be used 
for the solution. 



Note. — The contour problems in Figs. 33-38 inclusive are selected from 
a series of problems of this character used for instruction at the Army 
Service Schools, Fort Leavenworth, Kansas. 



112 MAP READING AND TOPOGRAPHICAL SKETCHING 

670 r- ■ — l870 



660 



8/0 



797 




785 



P/ace 10' Contours on the aboi/e Sketch 
The figures give ele vations in feet. ^ 



Fig. 35. — Contour Problem A. 



TOPOGRAPHICAL MAPPING 

917 960 962 



113 



930 



..„. -..900 Y . 

\ ,' 960 
969 k l 



960 



876 J 



918 



8^6' \ 



900 \ 



< i \ 

\ leap 

900 V6^tO 



-880 



: 840 
v >600 «' 840 



860 




9^0 



~6%2 678 

Place 20 ft Contours on above sketch 
Fig. 36.— Contour Problem £. 



i780 



f*«l 



/OJ5 /O/O , 



9/0 t ^ 3 ,< 




/»</>"> ?0 Contours 

Fig. 37.— Contour Problem C. 



114 MAP READING AND TOPOGRAPHICAL SKETCHING 



ebo . 




?~* 



Putin 20 'Contours. 
Fig. 38.— Contour Problem D. 



CHAPTER XII 
TOPOGRAPHICAL MAPPING— Continued 

Sketching from a Located Point 

Selection of Ground for Practice. 

In many cases there will be difficulty in selecting a suitable plot 
of ground for instruction in contouring. It is not necessary, 
however, to reject a piece of ground full of horizontal detail if 
it has the necessary variety of slopes, valley, spur, and ridge forms. 
For instruction purposes, the horizontal detail may be disregarded 
entirely so long as the buildings or trees are not sufficiently 
numerous to prevent getting a good view of the area. The worst 
possible condition of affairs is a piece of ground covered with thick 
brush or woods, and it should not be considered for instruction 
purposes, because it is impossible to get any view of any extent 
and consequently impossible to point out satisfactorily or prove 
the students' errors. A wooded area may, however, be used late 
in the fall or early in the spring when there are no leaves on the 
trees. 

With these considerations in mind, an area of ground con- 
veniently located is sought, about 1 square mile in extent, and 
possessing the best available characteristic ground forms. Other 
smaller areas may be used for preliminary problems. 

Scale and Vertical Interval. 

It is best to carry out the preliminary instruction of a student 
in sketching on a scale of 6 inches = 1 mile. Some favor the use 

115 



116 MAP READING AND TOPOGRAPHICAL SKETCHING 

of 12 inches =1 mile for preliminary instruction, because of the 
smaller amount of generalization involved. It is thought, however, 
that the student might as well face this problem of generalization 
at the start. The 12 inches =1 mile scale slows down the work 
too much. The 6 inches = 1 mile scale gives a far better idea 
of proper progress. 

Control Traverse. 

As a preliminary to the work of contouring, the student should 
run a control traverse through the area, closing and checking 
upon itself. This control traverse gives an excellent opportunity 
to show the student how much reliance can be placed upon the 
horizontal location of points. If the standard equipment is 
available, the plotted positions of the points of the traverse will 
be out of position by little more than the plotting error of the 
scale. The ground points located by this traverse should be 
remembered by the student. From what has been seen before, 
it is essential that the traverse shall furnish all the data for plot- 
ting the stream courses and drainage lines accurately. All is 
now ready for the delineation of the contour forms from the 
located points, and the student should be reminded that the only 
difference between what he has done and a formal survey lies in 
whatever inaccuracy there may be in his location of points, and 
that he now has the same opportunity to represent the ground 
faithfully from any given point as has the topographer in a formal 
survey. 

Analyzing Ground Forms. 

As a preliminary to the work of delineating the small area about 
a located point, the student should see accurately just what there 
is to be shown. If he is at the top of a summit, he must look 
for the beginnings of spurs and valleys, and note that the direction 
of these features is of importance. If in a valley, the character 
and degree of slope at the sides of the valley should be noted, 



TOPOGRAPHICAL MAPPING 



117 



also the direction of any minor spur or drainage line which would 
affect the contour forms. If among the indistinct ground forms 
lying near the crests there are any features that may serve to 
mark or distinguish the point, such as the beginnings of valleys or 
spurs, they should be noted. In other words, at any point from 
which sketching is to be done, the student should ask himself 
what he would have a right to expect to find on a good map of the 
area immediately about him, and if there is any feature which 
belongs on such a good map, then the student must be prepared 
to show it in its correct form and position upon his own sketch. 
Specific Example. Fig. 39 (1). In order to illustrate what is 
done at a single point, it is best to consider the procedure at a 





(/) (2) 

Fig. 39. — Sketching from a Point. 



point marked by characteristic ground forms. Such a point is 
represented at a. 

With the board set up and oriented at a, elevation 435 feet, 
inspection shows that there is a spur running in the direction of b, 
the crest of which is sharp, and that it changes slope at some 
point, as b', not yet determined; that in the direction c there is 
just enough in the way of a spur that the contours between the 
lines c and b are concave rather than convex in plan; that in 



118 MAP READING AND TOPOGRAPHICAL SKETCHING 

the direction of d, the axis of the ridge is level to some point d', 
and that the spur changes slope at some point d" , both points 
not yet determined; that in the direction e, this knoll at a is 
joined to other high ground by a saddle; that the contour forms 
between the lines d and e begin to show a valley ; and that in the 
direction of / is a rounded depression such that the contours are 
nearly semi-circular in form. This analysis of the ground is made 
by walking around from a, as may be necessary, and the result 
is to come to a definite conclusion as to just what is to be shown 
about o. 

Measurements from the Point. Fig. 39 (1). Having determined 
just what is to be represented, the necessary measurements are 
evident. First the lines b, c, d, e, and/ are plotted. Refinement 
in making these pointings with the alidade is not necessary. 
In a general way, it is sufficient to point the alidade as accurately 
as possible without stooping down to the level of the board, since 
nothing depends on these shots but the plotted direction of the 
axes of the features. In the direction of b the slope is now read 
for the first part of the slope, and found to be 5 degrees. Pacing 
in the direction of b, the 5-degree slope ends at 30 strides at 
b'. Here the slope is read in the direction of b, and found to be 
8 degrees, certainly as far down as the fourth contour below b. 
Returning now to a, after going 5 strides in the direction of c, 
the slope is measured and found to be 12 degrees, for at least four 
contour intervals. In the direction of d the axis of the ridge is 
level for a distance of 20 strides to d' '. Then begins a slope of 
6 degrees, to the point d", 50 strides from a. At d" begins a 9- 
degree slope, extending down at least three contour intervals. 
In the direction of e, a slope of 2 degrees is read and the distance 
52 strides paced to e', which is the low point in this saddle. Its 
elevation is determined to be 426 feet. After striding 8 strides 
in the direction of/, a slope of 12 degrees is read and mental note 
made of the semi-circular form of the contours in this direction. 
The result of all of these measurements is a series of contour 



TOPOGRAPHICAL MAPPING 119 

points along each of the lines b, c, d, and /. It now remains to 
draw the contours through these points in such form as to follow 
faithfully the ground before us. The slopes between b and c, and 
c and d, are both concave in plan rather than convex. Drawing 
the contours according to what we have observed, the result 
is shown in Fig. 39 (2) . 

Checking Results. 

After completing the delineation of the ground form, the 
result should be checked to see whether the representation is 
correct, showing the break in the slope in the directions of b 
and d, the very blunt spur in the direction of c, the saddle in the 
direction of e, the beginning of the valley between d and e, and the 
semi-circular form in the direction of /. If the representation 
answers all of these tests it is not likely to be the subject of criti- 
cism. The delineation above described is made to depend upon 
measurements, and a clearly marked hill form was selected to 
be delineated. It is not always possible to do the sketching in 
this way. 

The Level of the Eye. 

As soon as we begin to sketch by estimation, it becomes 
important to be able to select without instrumental aid a point 
on the ground at the level of the eye, and then by swinging the 
extended arm to describe a level line so that a satisfactory con- 
ception can be gotten of the trend upon the ground of a contour 
at the level of the eye. The ability to select, without instrumental 
aid, a point at the level of the eye is not a natural faculty but is one 
which must be carefully cultivated. Experience shows that with 
beginners in topography, if they are asked to point out a spot 
at the level of the eye, the average man will designate a point 
the actual reading on which is about plus 3 degrees. However, 
this tendency can be overcome by practice. 

It is possible, with a little practice before the mirror, to extend 



120 MAP READING AND TOPOGRAPHICAL SKETCHING 

both arms in a horizontal position. If the body be swung upon 
the hips, eyes, to the front, the hand will come into the line of 
vision on a level with the shoulder. If the thumb be turned up 
and fully extended, it will approximate to the level of the eye. 
Swinging the arms backward and forward in front of the eye in 
this position will give a good conception of the trace of the contour 
on the level of the eye. After considerable practice has been had, 
it is necessary to swing only one arm, and the result achieved 
will be quite satisfactory. 

Sketching by Estimation. 

Flat Hilltops. If instead of the clearly marked forms in 
Fig. 39, the hilltop had been comparatively flat, and the forms 
ill-defined, all we should have had to go on would have been an 
elevation of 435 feet at the location of the instrument, with 
nothing definite to read on in any direction. We will suppose 
that in a certain direction, after a distance of 50 strides, the 
ground begins to slope downward. We may plot that direction, 
and then, proceeding along that line, establish ourselves at a 
point such that the ground at the sketching board will be at the 
level of the eye. The clinometer might be used for this work, 
but it takes too much time. We are now standing on ground at 
430 feet elevation. Mark this 430 point and proceed farther 
until the eye is on a level with it. Swinging the arm will now 
sweep the 430-foot contour on the ground. The total distance 
to the sketching case is known, having been paced in coming out, 
so that both the plotted position of the point occupied, and a 
direction line will be on the sketch to guide in drawing the con- 
tour. Its meanderings for 50 strides each side of the line with 
respect to the line to the sketching case should be noted for use in 
plotting the contour upon return to the sketching case. About 
three or four outward journeys like this may be necessary to get 
a satisfactory delineation of the 430-foot contour. The total 
time necessary is not over ten or fifteen minutes and is well spent, 



TOPOGRAPHICAL MAPPING 121 

because the correct working out of the form of the 430-foot con- 
tour will then form the basis for all the ground forms lying below. 

Valleys. The valley contours lend themselves readily to the 
method of sketching by estimation. If we occupy a point on 
one side of the valley, we then judge the level on the other side 
of the valley to see whether the contour is .symmetrical on the 
two sides of the stream, estimate or pace the distance to the stream, 
and thus establish two points on the contour. Swinging the arm 
at the level of the eye, the point is estimated at which the con- 
tour crosses the stream. This point is estimated in distance by 
comparison with the width of the opening across stream. As 
has been indicated, the beginner will be surprised at how close 
the contour is on the other side of the valley, and how far up- 
stream it extends. 

Hillsides. In sketching from a point located on the side 
of a hill, the slope up and down may usually be read, and the 
contour spacing and locations in these directions established. 
Trace the contour at the level of the eye to see the form which it 
follows, what indentations and other peculiarities there are in it. 
The form of this contour will guide the drawing of the map con- 
tours at the point. Always, before leaving a point, examine 
the sketch to see if any feature is omitted that should be shown; 
to see that every feature that is shown is in its proper direction 
from the traverse point; and in general, that the representation 
of the ground is as accurate and as detailed as it lies within 
your power to make it, subject to the limitation that it is no use 
to spend time making little sinuosities and indentations in the 
contours that do not give any definite and specific information 
about the slope of the ground and would merely be lost in the 
general use of the map. In this connection, it is well to remem- 
ber that the slope of the ground at any point requires the contours 
both above and below it to completely show the character of the 
slope. 



CHAPTER XIII 

TOPOGRAPHICAL MAPPING— Continued 

Estimation of Distances, Mounted Sketching, Etc., 

Lettering 

Estimation of Distances. 

It is very important for the sketcher to cultivate the ability 
to estimate accurately distances up to 50 strides. This faculty 
can be acquired with little difficulty, because practice can be 
had at any time the student is walking around. Objects ahead 
are selected and the distance estimated. This estimate is then 
checked by pacing the distance. 

Until the ability to estimate distances accurately has been 
gained, it is well upon occupying a station where it may be 
desired to estimate distances, actually to pace the distance to 
some object about 8 or 10 strides distant. Then all distances 
to be estimated can be compared with this known shorter distance 
as a check. 

Having learned to estimate distances on the ground accurately, 
it is next desirable to be able to set off on the map distances up 
to 50 strides without the use of the stride scale. It will be found 
that this can be done with satisfactory accuracy after a little 
practice. 

The student should practice until able to estimate distances 
accurately on the ground, i.e., within 10 per cent as a limiting 
error and within 5 per cent as a rule, and until able to set off 
these estimated distances by estimation on the sketch with the 
same degree of accuracy. 

122 



TOPOGRAPHICAL MAPPING 123 

The student should learn to check constantly on the shapes of 
his map contours by mental comparison with the direction and 
estimated distance to points on the contour on the ground. Such 
comparison will serve as an effective guard against any serious 
errors in representation. Drawing contours as a result of deliber- 
ate and practiced estimate is a very different thing from merely 
drawing lines by guess and calling them contours. 

Speed in Sketching. 

In military sketching, speed is a very desirable quality, but it 
should be cultivated only after a satisfactory habit of accuracy 
has been acquired. 

Speed in sketching results principally from making every move 
count, and from moving all the time. The methods of setting 
up and orienting the sketching board should be noted carefully, 
for they are the methods involving the minimum of lost motion. 

The next item in time saving in sketching is the ability to 
decide quickly upon the exact work to be done at each station, and 
a definite plan of work. Make all the necessary pointings with the 
alidade, and when these are finished read all the vertical angles 
with the clinometer. 

The third item in speed is the substitution of rational and 
practiced estimate for actual measurement in the location of points 
upon which nothing else depends. Estimate must never enter 
along the main traverse. 

There is one time-consuming item against which we may 
as well be on our guard from the start, and that is the unneces- 
sarily short shot on the traverse. The general rule is to get as 
far along at each shot as possible, generally as far along as you 
can see. Every unnecessary set-up of the sketching board means 
a loss of several minutes. 

The beginner is apt to assume that, if his traverse lies along a 
road, his line of sight must lie within the road itself and hence 
sights on the first bend. The sight should go as far along the road 



124 MAP READING AND TOPOGRAPHICAL SKETCHING 

as is reasonably possible. In sighting to a bend where the road 
disappears from view, always sight on the inner side of the bend 
so that the sight may be prolonged as far as possible. 

Having sighted forward at a point such that a bend inter- 
venes, it is necessary to pace the distance around the bend of the 
road itself, whereas the distance desired for plotting is that along 
the chord of the curve. It will be sufficiently accurate in practice 
to estimate in strides the maximum offset from the chord at the 
bend, and deduct one-fourth of this number from the distance in 
strides around the curve. 

Mounted Sketching. 

Mounted sketching is usually understood to mean road 
sketching. In any case, the use of the horse confines the meas- 
urements of distances to roads. 

In one sense the term " mounted sketching " is a misnomer 
in that only the measurement of distances is done mounted. 
The sketch itself is drawn while dismounted, for obvious reasons. 

It is usual to assume that the sketcher has a mounted assistant, 
who takes and records the time between stations and holds both 
horses while the sketching is being done at the halts. It is per- 
haps better for the sketcher himself to take the time records, 
because he knows what data he wants. If he has no assistant, 
he must himself hold the horse at the stations. This is done 
preferably by standing on the reins. If the reins are held on the 
arm, a toss of the horse's head is annoying. 

The methods of mounted sketching differ very considerably 
from those used in dismounted sketching. The prime object 
is to get a traverse of the road, with such incidental information 
concerning the topography along the road as it is possible to 
procure. 

The traverse itself is made by making forward shots on selected 
points, reading the vertical angles, and plotting the distances 
between traverse stations from the time scale of the horse. 



TOPOGRAPHICAL MAPPING 125 

The data for plotting houses, bridges, stream courses, crests 
of ridges, boundaries of woods, etc., is recorded on the time pad, 
which is conveniently ruled so that the tune at which an object 
is passed and its distance from the road can both be indicated 
by a mark. Each page of the sketching pad is arranged to hold 
the memorandum record of five minutes on the time scale. 

The distance between stations is covered at a trot, and the 
stations must be an average of ^ to \ mile apart if proper progress 
is to be made. The stations are selected on the crests of ridges 
if practicable, so that the sketcher may look back over the ground 
to assist in drawing its representation. Otherwise the notes 
and the memory must be relied upon. 

Although a fairly satisfactory traverse can be run by mounted 
sketching, it is needless to remark that the representation of ground 
forms and elevations from such data and impressions as can be 
gathered while passing along the road at a trot, is only a very 
crude approximation. 

The usual mounted road sketch is executed on a scale of 3 
inches =1 mile, (R.F. 21 ,i ao ), V.I. 20 feet. The contours are 
supposed to show a strip of country 400 yards wide on each side 
of the road. At important positions, the contours may be extended 
further and they must of course be omitted where the ground is 
not visible from the road. 

The utility of contours drawn within the limits prescribed 
and by the means available in road sketching is very doubtful. 
However, the indications from the road sketches of adjacent 
roads may give a clue to the trend of the intervening ground. 

Trench Sketches. 

It will be frequently necessary to make sketches showing the 
lay-out of trenches, dug-outs, and the many auxiliary construc- 
tions. In such sketches, only a plan will be required as a rule, 
and the necessary view to show the contours may not be possible. 

The difficulties in running the traverse for such a sketch will 
lie in- the short shots and the presence of a great deal of metal. 



126 MAP READING AND TOPOGRAPHICAL SKETCHING 

The short shots require the adoption of a very large scale, 
and the presence of metal requires orientation of the sketching 
board by back sights instead of by the needle. 

Orientation by back sight is accomplished by placing the 
alidade on the line drawn for the forward shot and then turning 
the sketching board until the alidade points back along this line 
on the ground. The sketching board is then parallel to its pre- 
vious orientation. 

Lettering on Sketches. 

The topographical draftsman must be able to execute the 
several different Roman and Gothic alphabets required by con- 
vention for lettering different classes of information on finished 
topographical maps. 

The topographer requires no more than the ability to letter 
names and number contours neatly and plainly. The best form 
of lettering for his use is that in which a satisfactory degree of 
proficiency can be acquired with a minimum of practice. 

The alphabet which meets this* condition best is the single- 
stroke Gothic generally known as the Reinhardt alphabet, in 
honor of Mr. C. W. Reinhardt, who first systematized its 
construction. 

Alphabets. The Gothic italic alphabet, formally executed 
and giving the direction and order of strokes, is shown in Fig. 40. 
This alphabet should be studied carefully as to form of letters 
and general style. The order and direction of strokes in the 
vertical Gothic alphabet are the same as in the italic. Both of 
these alphabets should be learned. 

It is best to aim at reasonable speed in lettering combined with 
fair quality rather than to fall into the habit of labored lettering 
in the attempt to secure the best possible quality. Fig. 41 is an 
example of the former kind of lettering. The plate for this figure 1 
was made in twenty-eight minutes. The vertical Gothic letter- 



TOPOGRAPHICAL MAPPING 



127 



ing was executed in pencil and then in ink; the italic Gothic 
directly in ink. The plate is reproduced without reduction. 





*SGT&&0 



fg, fa fj'k 'him W -op <$■ 



Fig. 40.— Gothic Italic Alphabet. 

The letters of the Italic Gothic alphabet have a slope of 2 to 5, 
i.e., two units measured horizontal^ and five vertically downward 



128 MAP READING AND TOPOGRAPHICAL SKETCHING 

from a point serve to establish the slope line. The height of the 
bodies of the small letters is two-thirds that of the capitals. 

ABCDEFGHIJKLMN0PQR5 
TUVWXYZ 1234567890 
a bcdefg h ij k I m n opq rsfu v w 
xyz Vertical Gothic. 

A B CDEFGH IJKL MNOP. 
QRSTUVWXYZ 

123^567890 
abcdefghijk/mnopqrs t 
u v wxyz Italic Gofh/c. 

NEW YORK. New burg. 

Fig.^pl. — Vertical and Italic Gothic Alphabets. 

The basis of all letters is the straight line and an ellipse. 
These are vertical in the vertical alphabet. The straight lines 



TOPOGRAPHICAL MAPPING 129 

have the 2 to 5 slope in the italic Gothic and the major axis of 
the ellipse has a slope of 45°. 

Hints for Practice. Lettering is done with a finger motion. 
A firm and comfortable position of the hand is necessary. The 
pen or pencil should be held in about the same position as for 
ordinary writing, care being taken not to cramp the hand by 
squeezing the pen or pencil. 

For purposes of practice, the letters should be divided into 
groups. 

(1) Those formed entirely of straight lines: 
1, t, i, k, v, w, x, z. 

(2) Those involving the ellipse : 
o, a, c, e, s, d, g, p, q, b. 

(3) Those formed by a combination of straight and curved 
strokes : 

r, h, m, n, f, j, y, u. 

These groups should be practiced in order as given. The form of 
each letter should be learned from Figs. 40 and 41. Careless 
practice in lettering is time worse than wasted. Nothing is 
gained by repeating line after line of letters carelessly or wrongly 
formed. 

Speed should be increased gradually as proficiency is attained 
in forming the letters. The strokes that make up a letter should 
be executed in a regular cadence. The cadence should be gradu- 
ally increased. After the form of the letters has been firmly 
fixed in the mind and reasonable proficiency and speed in 
execution gained, practice should be undertaken in grouping 
letters into words, and then words into sentences. 

As a general rule, the space between letters should be approxi- 
mately one-quarter the height of the letter, and the space between 



130 MAP READING AND TOPOGRAPHICAL SKETCHING 

words about one and a half times the width of the letter n. The 
space between sentences is greater. It is best to learn to rely 
upon the eye to govern the spacing. All rules for spacing are 
subject to exceptions, and in many cases the spacing must be 
modified to conform to the margin. What is desired in the end 
is lettering so spaced that the eye is not offended by any unduly 
wide or narrow space at any point. 

Lettering in pencil is best accomplished with a 6H pencil 
sharpened to a very fine conical point and used with a very light 
touch. The test of all pencil lettering is that it must be possible 
to erase it cleanly and quickly with a soft rubber eraser. 

In lettering in ink, the weight of the stroke should increase with 
the height of the letter. This requires the use of a pen point 
suited to the work in hand. Gillott's 659 (crow-quill) or 170 
may be used for fine line work, and Gillott's 303 or 404 for medium 
weight lines. Hunt's Bowl-Pointed 512 or Leonardt's Ball- 
Pointed 506F give a weight of line suitable for lettering about 
\ inch in height. 

Aptitude is a valuable asset in learning to letter, but pro- 
ficiency is within the reach of all who will study the forms of the 
letters carefully and then devote a reasonable time to careful 
and systematic practice. 



CHAPTER XIV 
PANORAMIC SKETCHING 

Purpose. In the identification and assignment of targets for 
both field artillery and machine-gun fire, it is desirable to record 
the positions of the targets in some form such that they may 
be identified easily and unmistakably. For artillery observation 
points, it is desirable to be able to identify points of special enemy 
activity and identify them so that a record can be made for the 
use of successive reliefs. 

All of these purposes are best fulfilled by a panoramic sketch, 
which is nothing but a conventionalized perspective landscape 
sketch, with certain data entered upon it with reference to the 
azimuth and range to targets from the point where the sketch was 
made. 

Perspective. 

In order to make a panoramic sketch, some knowledge of the 
laws of perspective is necessary, particularly in regard to the 
vanishing points of parallel lines. From the standpoint of the use 
of the sketch, it will serve its purpose if the points in the landscape 
can be identified, even though the perspective be comparatively 
crude. Many of these sketches, in practice, are nothing more 
than approximate horizon outlines with a few of the intervening 
sub-crests. 

Crest Lines. 

The first step in the drawing of a panoramic sketch is to draw 
the approximate representation of the crest lines. The land- 
scape must be studied carefully in order to pick out the lines of 

131 



132 MAP READING AND TOPOGRAPHICAL SKETCHING 

the sub-crests and the hill over-laps, because upon the rendering 
of these few lines depends much of the effect of the sketch. These 
crest lines should be drawn in the lightest possible line so that 
corrections can be made easily, until the entire outline is satis- 
factory. The aids to the correct drawing of the crest lines will 
be described in connection with the sketching pad. 

Distance Effect. 

The sketch should, in so far as the skill of the sketcher permits, 
give a correct impression of the landscape and, therefore, should 
give the impression of increasing distance as the objects represented 
are farther from the point where the sketch is made. This is 
accomplished by making the outline of the distant horizon in a 
faint, sketchy and discontinuous line. The lines of the sub-crests 
are then touched up in a gradation of lines, depending upon the 
distance, so that the nearest objects shown in the sketch are in 
bold, black strokes. Cleared hills are shown in smooth outline, 
wooded crests by an irregular outline made up of small loops 
like the tree symbol in Fig. 10. 

Full advantage is taken of the effect of the convergence of par- 
allel lines in perspective to strengthen the effect of distance. 

Conventions. 

The representation of the various features of the landscape is 
more or less conventionalized. The representation of roads, 
buildings, trees, clumps of woods, etc., is shown in Figs. 44 to 46, 
inclusive, at the end of this chapter. 

Sketching Pad. 

Two forms of sketching pad are provided in our service, one for 
the use of infantry, the other for the use of the artillery. They 
do not differ materially except that the artillery pad has addi- 
tional ruled spaces at the top of each sheet for the artillery firing 
data. The School of Musketry sketching pad is shown in reduced 
size in Fig. 42. The sheets of the pad are 8^ by 5| inches. Verti- 



PANORAMIC SKETCHING 



133 



< 




























































































1 

S? 
o 

•o 

c 
















ai ■ <u v 
c " £ C 

1- D 2 as 




















































































<Y 


Z 


L 
Q 














a, <5 o 








134 MAP READING AND TOPOGRAPHICAL SKETCHING 

cal lines in light blue are ruled on the paper. These vertical lines 
are used in dropping lines from the landscape to their positions 
on the sketch. The f-inch spaces between these vertical lines 
subtend an angle of 50 mils (1 mil= ^~ of a circle) when the 
pad is held at 15 inches from the eye. To insure this distance, 
a cord is secured to the pad and knotted at exactly 15 inches. 
The knot is held between the teeth in measuring the deflection 
of objects in the landscape. 

Four horizontal lines are ruled at intervals of \ inch just below 
the center of the sheet to mark the vertical limits of the sketch. 
At the top of the sheet are two orientation marks, and horizontal 
lines giving spaces for the description of the target (T), the range 
(RN), and the deflection (DF) in mils from the reference point. 

Practice in Panoramic Sketching. 

The beginner will be confused by the mass of detail in the 
landscape before him. All but the bare outlines of the features 
must be ignored. Prominent objects in the foreground are 
omitted entirely, because they are of no importance in the use 
of the sketch. 

It is best before attempting to make sketches from the land- 
scape to practice copying typical sketches, in order to become 
accustomed to the representation of objects in the distance, and 
to the extremely small size of the objects that are drawn in the 
sketch. The next step is the drawing of imaginary landscapes, 
after which sketching from a landscape should be attempted. 

Making the Sketch. 

The sketching pad is held in front of the eye, the upper edge 
of the paper horizontal, the cord knot in the teeth. The pad is 
shifted until the desired sector is included between the orienta- 
tion marks. The points in the sky-line now appear at the edge 
of the pad vertically over the points where they are to be drawn. 
These points are marked at the edge of the pad. The pad is now 



PANORAMIC SKETCHING 135 

lowered, and the highest point on the sky-line is indicated in its 
proper position on the highest of the horizontal lines of the sketch- 
ing area in the center of the pad. Other points of the sky-line 
are now drawn in their proper vertical and horizontal positions. 
The sky-line is drawn in lightly. Other points, crests, targets, 
etc., are then entered in the same manner, again holding the pad 
up before the eye if necessary. 

By this method the objects will be drawn in their proper lateral 
positions. The vertical proportions should be so determined 
that the sketch will lie between the limiting lines. This may be 
done by holding the pad up, and lowering it by four equal stages, 
so as to show what ground features belong on each of the hori- 
zontal lines of the sketching pad. 

In making a sketch, no effort should be made to obtain artistic 
effect, and it is necessaiy to avoid too much detail or the sketch 
will become confused. Outlines only are shown. 

A prominent object is selected as a reference point. This must 
be an object easily recognized by anyone using the sketch. A 
vertical line is drawn to this point and a heavy arrow-head placed 
on the line near the object. The line is labeled in the deflection 
space. The name or description of any target is indicated in the 
target space, the range in the range space, and the deflection in 
mils from the reference point to points indicated by vertical lines 
drawn from the objects up through the deflection space. When the 
range is measured by a range finder, the range entered is under- 
scored; when estimated, not. 

Panoramic sketches must be done in the shortest possible 
time. Ten minutes is the usual time allowed for completion. 

When a sketch is completed, the pad is held in the position in 
which the sketch was made, then turned downward into a hori- 
zontal position, the compass north determined by placing the 
compass on the pad, and a north line is drawn at the circle at the 
lower edge of the pad. 

An example of the completed sketch is shown in Fig. 43. 



13G MAP READING AND TOPOGRAPHICAL SKETCHING 




PANORAMIC SKETCHING 



137 




Sky/jne fa Crej/s. 




Roads 




Lone Trees. 




Woods in Relief. 

Fig. 44. — Panoramic Sketching — Conventions. 



138 MAP READING AND TOPOGRAPHICAL SKETCHING 





Pole' Lines. 




Railroads. 

Fig. 45. — Panoramic Sketching — Conventions 



PANORAMIC SKETCHING 



139 




Villages 




Buijdings. 




Woods behind Crej/j. 

Fig. 46. — Panoramic Sketching — Conventions. 



ROUND ISLAND 



WESTON SHEET <f 



& 



6515 




4117 



4115' 



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6515 



Illustrate Topographical Drawing. ^ 

April, 1917. 



■-> 



SPECIAL. SURVEY 



ROUND ISLAND ^ 

WESTON SHEET ^ 



4l'l 7 




"65 18 

■f C D. Chief Topographer. 

J? Tr.engulation by E F 

ylr Topography by G. H .and I M 

Surveyed in 19. 



To Illustrate Topographical Drawing. 
April. 1917. 



Contour interval 10 feet. 
Datum mman sea levml. 

PLAT« A. 



LIBRARY OF CONGRESS 




011 392 985 6 



